LED Sensitivity Survey: Discussion

Last update: Analysis of November 27, 2021 survey data

Number of survey responses = 12; 11 with LED symptoms

The graphs on the page below will not automatically update as more survey responses are made - they are the graphs from the most recent archived copy of the survey response spreadsheet. Graphs on the "Graphs" page of this website will automatically update as more people respond to the survey.

Current survey results spreadsheet

Current LED Sensitivity Survey Questions (PDF)

Archived survey results

Archived LED Sensitivity Survey Questions (PDF) - version archived 12/12/21


The LED Sensitivity Survey is an attempt to begin to collect the experiences of individuals with sensitivity to LED lights and/or screens with the intention of informing the public, the lighting and computer industries, the medical community, and policy makers. While this survey is a survey of adults, possible effects on the health and learning of children should also be a significant consideration of future research.

A preliminary analysis of survey responses shows that respondents sensitive to LED lights and/or screens do not have the photophobia typically experienced by migraine patients, which is sensitivity to the brightness of any light, because respondents are not bothered by sunlight. Rather, their experience of light aversion is specific to the kind of light produced by LED lights or screens. Most survey responses are consistent with flicker, rather than blue light or light brightness, being the main noxious stimulus, although some contribution of blue light remains a possibility. Respondents tend to report multiple symptoms that may include headache, pain in or near the eye, eye muscle pain, dry eye, spatial disorientation, nausea, scalp allodynia (sensitivity to normally innocuous stimuli), concentration and short-term memory problems, sleep abnormalities, anxiety, depression, and various additional symptoms that suggest changes in the nervous system due to sensitization. Sensitization is the permanent structural or molecular changes to neurons that enhances signaling or expands signaling to nearby neurons. Symptoms, while partially overlapping, vary among individuals. Symptoms tend to also partially overlap with symptoms experienced by people with other disorders that involve photophobia, including dry eye, migraine, and concussion (mild traumatic brain injury), including longer-term symptoms that mimic longer-term concussion symptoms. Diel et al. (2021) recently argued that photophobia and other overlapping symptoms in dry eye, migraine, and concussion suggest that particular peripheral and central nervous system signaling pathways, sensitization, and neuroinflammatory processes may be common to and underlie symptoms of these conditions. The fact that symptoms survey respondents attribute to LEDs also overlap with symptoms of these other photophobia conditions suggests the possibility that LED light or screen flicker may trigger subsets of the same symptom-creating nervous system signaling pathways and/or neuroinflammation in sensitive individuals. If this is the case, it has implications not only for individuals whose primary complaint is LED sensitivity, but also for those recovering from concussion or experiencing migraine or dry eye, for whom LED flicker might not only exacerbate existing signaling problems, but may even trigger additional adverse effects.

What kinds of LED lights are unhealthy for sensitive people?

The overall survey results generally suggest that flickering lights cause symptoms and completely flicker-free lights do not cause symptoms for people sensitive to LEDs. Completely flicker-free lights include sunlight and LED lights that have been engineered to be completely flicker-free.

The health effects of LED lights have not yet been defined in the scientific literature (see Background: Health Effects). Possibilities for the aspect of LED light that has harmful health effects include the brightness, the color temperature, or the flicker of LED lights.

Respondents to the LED Sensitivity Survey are generally sensitive to light, meaning that they experience "photophobia." However, photophobia is a broad term that encompasses many different types of sensitivity to light, including pain or headache aggravation when experiencing light of normal brightness during a migraine, or sensitivity to some aspect of artificial light or screens for many patients with post-concussion syndrome (discussed further below in "All photophobia is not the same"). Survey responses provide insight into what aspect of LED light triggers health effects for sensitive individuals.

1. Could light brightness be the trigger of LED symptoms?

Light brightness is unlikely to be an explanation because no (0/11) survey respondents indicated that reading in sunlight triggered their symptoms (Fig. 58), although the brightness of sunlight does tend to be irritating during migraines in which the photophobia is sensitivity to light brightness (see Background: Health effects of flicker ≥100 Hz and the discussion below for descriptions of photophobia). In further support of light brightness not being the triggering factor, only 2 of 11 people indicated that sunglasses help more than "a little" in preventing their LED symptoms (Fig. 76).

It is complicated to interpret responses about screen brightness because different screens can use different dimming mechanisms, and many, but not all, screens will have more flicker when dimmed. Almost all (10/11) respondents indicated that setting a screen at full brightness is "irritating," but only four of these people indicated that this made their LED symptoms worse and two of these people said that setting the screen at full brightness helped to prevent their LED symptoms (Fig. 69). The others indicated the brightness had no effect on whether they developed LED symptoms. Overall, these responses indicate that survey respondents are irritated by full brightness screens, but there is disagreement over how screen brightness impacts LED symptoms. It's also possible that screen brightness might be more irritating during LED symptoms, even if it doesn't have a role in triggering the symptoms.

Most significantly, there is not support for sunlight triggering symptoms and almost no support for sunglasses being significantly helpful in preventing symptoms, suggesting that light brightness is probably not generally the trigger for people with LED sensitivity.

2. Could blue light be the trigger of LED symptoms?

Responses to multiple survey questions did not support the hypothesis that blue light is the aspect of LED lights that causes LED symptoms. 9 out of 10 people reported that both warm and cool color temperatures of LED lights cause their symptoms and one person reported only warm color temperatures (Fig 60). Only one person out of 10 reported that blue-blocking glasses were any more than "a little" helpful in preventing their symptoms from LED lights (Fig 77). Only one person out of 10 reported that blue-blocking lenses helped "a lot", but not completely, in preventing symptoms from LED screens (Fig 78). Of the 9 individuals who tried blue-blocking lenses, 7 reported that they didn't help more than a little to prevent their symptoms, 1 reported that they helped "a lot" when using screens, and 1 reported that they helped "some" with ambient LED lights (Fig 106)

Although sellers of blue-light blocking glasses promote the idea that blue light is the aspect of ambient light or computer screens that causes headaches or eyestrain, there is little support for that hypothesis in this survey, both in terms or a lack of connection between color temperature of light and symptoms and because of the fact that blue-blocking lenses do not help 7 of the 9 people who tried them (Fig 106), with the others reporting partial, but not complete prevention of symptoms.

However, the survey responses do not rule out the possibility that some individuals might experience adverse health effects as a result of blue light. Note that there is significant evidence in the scientific literature supporting a role for blue light in regulating circadian rhythms (see Background: Health effects of blue light).

Also see Self-tests for descriptions of tests that allowed me to personally rule out blue light as causing my LED symptoms.

3. Could flicker be the trigger of LED symptoms?

The data are most consistent with flicker being a primary cause of LED sensitivity symptoms. Without any special engineering, LED bulbs flicker dramatically when on AC power, turning completely off 120 times a second (in the Americas, or 100 times a second elsewhere), although this flicker is so fast that people are not usually aware that they are seeing it. LED bulbs can be engineered through the choice of extra bulb circuitry, to either have reduced flicker or to be completely free of any flicker. In the U.S., many completely flicker-free LED bulbs were manufactured in the past and a very limited number of flicker-free bulbs are being manufactured in 2021 (see Background: LED Lights). It is not yet known if there are kinds of flickering light that cannot be sensed by the brains of people who are particularly sensitive to flicker (see Background: Health effects of flicker ≥100 Hz).

Because flicker ≥100 Hz is so fast that people are usually not aware that they can see it, people may not realize that it could be a problematic quality of some artificial light. People also may not realize that they might try to use slow motion videos to detect some flicker and most people are very unlikely to have access to an oscilloscope or flicker meter, which is needed to detect flicker of high frequency or lower flicker percent. The inability to detect some flicker with a smart phone slow-motion video can also mislead people into falsely assuming that the lights that cause their symptoms don't flicker. For example, the LED strip lights that first caused serious symptoms for me had such subtle flicker that I often couldn't detect it with a smart phone and when I could, it was extremely subtle and I only learned with certainty that the lights flickered when a lighting analyst shared statistics about the lights (see Self-tests). This means that many people may not have done sufficient testing on their own to be able to share an informed opinion about the role of flicker in triggering their symptoms.

A major argument for flicker being a primary problem for people reporting sensitivity to LED lights and/or screens is that none of the survey respondent's symptoms are triggered by sunlight and with one exception discussed below, symptoms are not triggered by completely flicker-free LEDs for the survey respondents who have tested them. Sunlight and completely flicker-free LED lights are two kinds of light without flicker. None of the 11 respondents indicated that reading in sunlight triggers their symptoms (Fig 58). Additionally, 4 out of 6 people who used an instrument or camera to detect the flicker of LED light bulbs reported that their symptoms tended to be worse when there was greater flicker (Fig 62).

All three people in the survey who are sensitive to flickering LEDs and who have tried LED bulbs that they were able to confirm were completely free of all flicker reported that completely flicker-free LED bulbs do not cause their LED symptoms (Fig 64).

The contradictory report from one individual who indicates that their symptoms are only caused by completely flicker-free LED lights, but not by flickering LED lights (Fig. 64), and not by sunlight could have arisen for multiple reasons:

  1. The report may be as the individual intended and based on accurate information, in which case this would suggest that completely flicker-free LED lights have a harmful characteristic that is not shared by either flickering LED lights or by sunlight. Sunlight and flicker-free LED light are both continuous light sources, so the continuous nature of the light is unlikely to be the trigger for this person. Flickering and flicker-free LED lights do not differ in their color profiles, so the color of the light is unlikely to be the trigger for this person. The maximum light output of a flicker-free LED light is less than the maximum light output of a flickering LED light with the same average brightness (see Background: LED lights), although it seems unlikely that a lower maximum light output could produce negative health effects. Perhaps there is some other characteristic unique to completely flicker-free lights.

  2. The report may have been the opposite of what the individual intended and perhaps they meant to say that only flickering LED lights cause their symptoms. Perhaps there was something confusing in the survey language.

  3. The report may be as the individual intended, but based on inaccurate information. This individual said that for LED lights that caused their symptoms, they could not detect flicker themselves using a smart phone slow motion video. In my experience, a smart phone is insufficiently sensitive to detect some flicker and cannot be used as the sole source of information to distinguish between flickering light and flicker-free light (see Self-tests). This individual says that a representative of the lighting industry told them that the lights suspected in causing their symptoms did not flicker. It's possible that the light did flicker, but that the lighting industry representative was using the much more restrictive definition of flicker adopted by the lighting industry in 2016 (CIE TN-0060), rather than the definition used by the general public and used in the survey. The lighting industry's definition of flicker indicates that if a person can't knowingly see individual pulses of light, they won't call it "flicker," but instead call the flicker "temporal light modulation" (CIE TN-0060). Thus, since this individual couldn't consciously see any flicker, according to this industry-specific definition, it's quite possible that a lighting specialist would say that the lights don't "flicker" even if they actually do flicker. This is an example of how the CIE TN-0060 redefining of flicker might be misleading and confusing. Another possibility is that something else in the environment besides the lights were triggering the person's symptoms, although if this person's experience is anything like mine, it is quite obvious when LED lights are the triggering factor and I'm reluctant to suggest otherwise for someone else.

Together, these data suggest that flicker, rather than blue light or light brightness, is most likely to be the cause of LED symptoms, for most people in the survey. This hypothesis is also consistent with the high frequency with which people in this survey were sensitive to slower, obviously visible flashing lights (7/11), repetitive patterns (4/11), or either of these (8/11); see Figs. 10-12. Relatively slow frequency visibly flashing light is a known trigger in photosensitive epilepsy and in "flicker vertigo." People with photosensitive epilepsy are also often sensitive to viewing certain kinds of repetitive patterns (see Background: Health Effects of <100 Hz flicker). Sensitivity to certain kinds of visual stimulation like being in a crowd or looking at shelves in a supermarket has also been reported for visual vertigo (reviewed in Chin, 2018), although this seems to be a different kind of visual sensitivity than that associated with flashing lights or repetitive patterns.

Flicker has also previously been reported to cause headaches and eyestrain in scientific studies, as in studies of magnetic-ballast fluorescent lights. These effects are worse as aging of the lights creates quite visible flicker. People who get migraines are also more sensitive to obviously visible flicker than other individuals (see Background: Health Effects of <100 Hz flicker and Background: Health Effects of ≥100 Hz flicker for references and discussion).

Although there are well-known health risks from obviously visible flicker and from the 100 Hz or 120 Hz flicker of magnetic-ballast fluorescent lights, there have not yet been any studies to assess the health effects of the ≥100 Hz flicker of LED lights beyond a couple very limited studies of normal control individuals. It is hoped that relevant research will soon begin and will include the assessment of health effects in people sensitive to LED lights and/or screens.

It is more difficult to draw conclusions about which aspects of screens cause the health effects from LED screens because screen technology is even more complicated than LED light technology and screens include multiple potential sources of flicker (see Background: LED Screens).

Note: A caveat to understanding the effects of color temperature or light brightness is that both of these may not be independent from light flicker. For example, a common dimming method for LED lights is to increase the length of time of the dark phase of the flicker (pulsed-width modulation, PWM), thus making the flicker more evident to the brain. A bulb with a very bright "on" phase may appear to be dim overall if its flicker is pronounced, even though the light is actually very bright during the "on" phase of the flicker. White-tunable or color-tunable LED lights may create a precise color by flickering LEDs of different colors (see Background: LED Lights).

How common is LED sensitivity?

No one knows how common it is for people to experience health issues in response to LED lights or screens, and the LED Sensitivity Survey is not designed to answer this question. However, related research and anecdotal reports hint that the prevalence of LED sensitivity could be fairly significant.

Earlier studies of the health effects of flicker from fluorescent lights indicate that a significant fraction of the population is sensitive to lighting flicker. Brundrett (1974) surveyed 627 office workers in the UK and asked whether they experienced eyestrain, headaches, or fatigue while working under magnetically-ballasted fluorescent lights, which had both 100 Hz flicker and, to a lesser degree, 50 Hz flicker, especially in aging lamps. Very high rates of headache (45%) and eyestrain (40%) were reported by the office workers (see Background: Health effects of flicker below 100 Hz). In a double-blind study of fluorescent lights by Wilkins et al. (1989) administered in to 159 office workers in a UK government legal department in 1986-1987, magnetically ballasted fluorescent lights with 43-50% flicker at 100 Hz were linked to causing eyestrain in about 8% of people and headaches in about 8% of people. Most, but not all, of those people experienced both eyestrain and headache (see Background: Health effects of flicker ≥100 Hz).

It isn't clear how relevant these studies of the health effects of flicker in fluorescent lights are to LED lights. However, many LED lights sold, at least in the United States in 2021, have a higher flicker percent than the fluorescent lights in the Wilkins et al. (1989) study. This suggests that headaches and eyestrain from these LED lights would be expected, although the characteristics of LED waveforms differ from those of fluorescent waveforms, so it isn't know whether more or fewer health problems might be expected from LED lights with flicker similar to fluorescent lights.

Virtually nothing is known about whether most LED lights, especially those with lower amounts of flicker, should cause health problems (see Background: Health effects of flicker ≥100 Hz). Likewise, although there are reports in the scientific literature that screen use adversely affects the health of sensitive people, there isn't a clear sense of the prevalence of screen sensitivity, and the aspects of screens that are associated with sensitivity are incompletely understood (see Background: Health effects of flicker ≥100 Hz and Background: Health effects: More screen sensitivity literature). Some types of screen sensitivity have been described as "cybersickness" and some as "computer vision syndrome" or "digital eye strain." Many anecdotal reports also describe screen sensitivity (see Background: Health effects: Anecdotal reports).

In a large-scale study of headache among information technology (IT) workers (n=2012; 77% of respondents were male) in 2018 in Beijing, Li et al., 2020 found an elevated headache incidence overall, as well as a somewhat elevated incidence of migraine in females and a much more significant increase in tension-type headache (TTH) in both males and females. Their data was compared to an even larger study (n=5041) that their research group conducted of headache incidence in the general population in China in 2009 (Yu et al, 2012).

Data from these two studies are summarized below. Also see Background: Health effects of flicker ≥100 Hz.

While these data do not directly address the question of how common LED sensitivity might be, they sugest that such sensitivity could be quite common. This study might even under-report the incidence of headache because the highest incidences of headache were found among individuals who were particularly under-represented in their study: women between 30 and 50 years old. While job stress could also play a role in the much higher incidence of headaches among IT workers in 2018 compared to the general population in 2009, it is also quite likely that increased LED screen use and/or increased ambient LED light use in recent years and in this type of occupation might account for a significant amount of the increase in headache incidence. In support of this, Li et al., 2020 also show that excessive computer use (>8 hours/day) is a risk factor for TTH in their study. The study might also under-report how common LED light and screen sensitivity might be since those with particular sensitivity might not be able to actively work in this field.

Significantly, in further support of the hypothesis that LED lights or LED screen use might contribute to the high incidence of headache, Li et al. further report an unusually high percentage of people they classified as having tension-type headache (TTH) also reporting photophobia, as well as an unusually high rate of photophobia reported by respondents with any type of headache (75.8%). This abnormally high rate of photophobia was an unexpected anomaly in the data that they couldn’t explain (Li et al., 2020; see section V.3). They assumed that there must have been a mistake in how the description of photophobia was communicated to participants that led to over-reporting of the symptom. They decided to exclude photophobia from their diagnostic criteria because “the prevalence estimate for migraine would otherwise have been much higher” (Li et al., 2020). Thus by excluding photophobia from their diagnostic criteria, they artificially increased the proportion of TTH diagnoses and decreased the proportion of migraine diagnoses among the IT workers with headache. This likely explains why most of the increased headache incidence is classified as an increase in TTH. This same research group did not have a similar conundrum concerning diagnoses including photophobia in their 2009 study of the general population.

A high incidence of photophobia in the Li et al. study is quite consistent with the results of the LED Sensitivity Survey, which shows that those reporting sensitivity to LED lights and/or screens report headache symptoms that don't necessarily completely fit with traditional classifications of migraine or TTH. It is quite possible that the Li et al. study actually uncovered a novel category of headache triggered by LED light and/or screen use. Their data and data from the LED Sensitivity Survey suggest that LED light and/or screen use may cause a novel class of headaches characterized by photophobia. Unlike in other headache types, the light might be the trigger to cause the headaches, rather than the photophobia simply being a secondary symptom. This warrants further research.

LED sensitivity: More severe than "visual disturbance," "eyestrain", or "just a headache."

Survey respondents report symptoms with severe impacts on their physical health that compromise their ability to work, complete their education, or otherwise access environments with LED lights or use LED screens. Some, but not all, survey respondents also report mental health impacts.

Symptoms attributed to LED lights and/or screens in the LED Sensitivity Survey are wide-ranging. The most common or severe symptoms, as well as additional symptoms overlapping with symptoms from other photophobia conditions, are listed below:

  • pain in or around the eyes (10/11)

  • dry eyes (10/11)

  • photohobia (10/11, with 3/10 reporting sensitivity to any light and 7/10 reporting sensitivity only to artificial light)

  • headache (9/11)

  • pain or soreness in eye muscles (9/11)

  • anxiety (8/11)

  • visual disturbances (8/11, including blurry vision for 4/11 and peripheral blindness for 1/11)

  • several types of sleep disturbance (6/11)

  • fatigue (6/11)

  • concentration problems (6/11)

  • short-term memory problems (5/11)

  • nausea when viewing lights/screens (5/11)

  • spatial disorientation (4/11)

  • depression (4/11)

  • symptoms last over a week or never going away (4/11)

  • tinnitus (3/11)

  • phonophobia (3/11)

  • a scalp that's painful to the touch (allodynia; 3/9)

  • warmth or a burning sensation in the head (2/11)

  • seizures (1/11)

  • numbness in the body (1/11)

  • ongoing nausea and loss of appetite (1/11).

See figures 20-42 in Survey: Graphs for more comprehensive descriptions of LED sensitivity symptom frequency from the survey. Select symptom frequencies are also summarized in Table 1 and compared to the symptom range for other conditions with partially overlapping symptoms (Table 1 PDF that opens in a new window or see Survey: LED symptom categorization for a version of Table 1 that automatically updates):.

Survey respondents tended to report that their LED sensitivity symptoms severely impact their lives, with none reporting that their symptoms are "not bad," 6/11 reporting that symptoms are "quite bad", and 5/11 reporting that symptoms are "very bad" (Fig. 48) Additionally, they either cannot treat/control their symptoms at all (7/11) or only a little (3/11), with only one person saying they could treat/control their symptoms "quite well" (Fig 46). 10/11 report that their symptoms prevent them from doing at least some day-to-day activities (Fig. 49).

LED sensitivity symptoms impacted the education of 6/11 respondents, with 4 of the 5 who reported no educational impact having symptom onset after age 25 (Fig. 50). LED sensitivity symptoms have negatively impacted the careers of 8/11 survey respondents (Fig. 52), with 4/9 either wanting or requesting a workplace accommodation to avoid LED light exposure (Fig. 54) and 6/9 either wanting or requesting a workplace accommodation to minimize LED screen use (Fig. 55).

Of the 8/11 respondents who also experience other kinds of headaches that they don't attribute to LEDs (Fig. 84), 3/8 report that their other headaches are "not bad", 4/8 report they are "quite bad," and the one person with migraine reports that their migraine symptoms are "very bad." 7/8 of those individuals, including the individual with migraine, report that the headaches that they attribute to LEDs have much more impact on their lives than their other headaches (Fig. 85). In support of this, 7/8 report that they think they are able to treat/control their other migraine or tension type headaches "quite well", none think that their other headaches interfered with their education, and only 2/8 think that their other headaches have negatively impacted their careers. Together, these responses suggest that people reporting symptoms due to LED sensitivity are not prone to playing up the severity of their health problems in general. It is quite significant that they think that symptoms that they attribute to LED lights and/or screens have much more of a serious impact on their lives than even migraine or TTH.

Therefore, the symptoms associated with LED sensitivity have much more of a negative impact on people's lives than even migraine or tension-type headache (TTH). The inability to treat or control LED sensitivity symptoms, coupled with their diverse and significant neurological impacts, the often long duration of symptoms, and the increasing ubiquity of LED lights and screens in everyday life makes LED sensitivity a much greater challenge for affected individuals than would be implied by descriptions like "visual disturbance," "eyestrain," or "just a headache."

Personally, I consider myself incredibly fortunate to have been able to complete my education long before LED lights and screens were produced. My brain simply would not have been able to function well enough to manage even the most basic subjects in school if classrooms had LED lights or if learning depended upon use of an LED screen (see Self-tests). A PhD in genetics would have been an impossibility.

The severity of LED sensitivity symptoms currently reported may be the tip of the iceberg, since the most severely affected individuals may not have wanted to undergo the screen time necessary to complete the survey in the first place.

Do people reporting LED sensitivity have one common syndrome or multiple different syndromes?

Data are currently limited, with only 11 responses. While there are differences, there are also commonalities among responses. Although more data are needed to answer the above question, the partial overlap among survey responses and the significant overlap of LED symptoms with symptoms of migraine, concussion, and/or dry eye disease hint that there may be common neurological pathways underlying individuals' symptoms.

The data are currently too limited and the amount of symptom variation among individuals is too great to draw a conclusion about whether sensitive individuals have variations of the same syndrome or experience multiple distinct syndromes. However, even with only 11 responses, it's clear that there is significant overlap of symptoms among individuals who have responded to the survey to date. Tree 1 clusters individuals' symptoms based on their similarity and there is significant, but not universal, overlap of symptoms among individuals. Many have pain associated with eye muscles/focusing, many have pain/pressure in or near their eye(s) or headache that doesn't seem related to eye muscles or focusing. Many experience spatial disorientation and/or nausea, Many have short-term memory/concentration problems. Many have sleep abnormalities. More survey responses could help to more clearly define whether there are distinct groups of symptoms for different subsets of individuals.

Interestingly, both the common and the more rare symptoms reported in the LED Sensitivity Survey overlap with symptoms of the constellation of dry eye disease, migraine, and concussion, for which common underlying neurological pathways have already been suggested by Diel et al. (2021). When constructing the survey questions, I was aware of the symptoms of migraine due to experiencing common migraine myself and I was also aware of the general issues for patients with ongoing concussion symptoms due to having been a high school teacher and reading educational materials from the school nurse and students' doctors when multiple of my former students had sports-related concussions. However, I was not aware of the spectrum of dry eye symptoms and was unaware of many of the secondary symptoms that can occur in migraine, concussion, or dry eye due to sensitization of the nervous system, including allodynia, feelings of burning/warmth, numbness, tinnitus, and sometimes neural signaling that leads to anxiety or depression. For the survey, I included all of the symptoms that I found reported for 79 sensitive individuals on LEDstrain.org (see Methods) whether or not their symptoms made sense to me at the time. It's interesting in retrospect, now after having been able to review the literature, that some of the symptoms that I initially thought were some of the more surprising effects of LED light are in fact also symptoms of dry eye disease, migraine, and/or concussion. Those symptoms indicate that nervous system signaling has changed due to sensitization, suggesting that nervous system sensitization might also occur in LED sensitivity (see below).

Also see the previous section for an outline of symptom frequency.

Are LED-caused symptoms novel or an example of a disorder previously described in the medical literature?

The collection of symptoms reported in this survey seem to be novel (not yet reported in the medical literature), although many of the symptoms are widely reported in online anecdotes. The symptoms associated with LED sensitivity seem to be a new phenomenon because (1) most people's symptoms only started after the introduction of LED lights and screens and (2) there isn't another condition that fully overlaps the symptoms of LED sensitivity.

The symptoms reported by individuals in the LED Sensitivity Survey are something new that correlates with the introduction of LED lights and screens. The year of symptom onset is 2011 or later for 10 of 11 respondents, with the remaining person reporting symptoms since 2007 (Fig. 4).

Possible diagnoses already described in the medical literature with partial symptom overlap with the symptoms in the LED Sensitivity Survey are headaches, including migraine or tension-type headaches, dry eye disease, post-concussion syndrome, as well as eyestrain/asthenopia or Computer Vision Syndrome, which have symptom similarity with dry eye disease. Photophobia can be a component of all of these conditions. However, none of these conditions fully fits the scope of the symptoms reported by those with LED sensitivity.

Additionally, there is partial symptom overlap of LED symptoms with flicker vertigo, as described by professionals associated with aviation (see Background: Health effects of flicker below 100 Hz), and with cybersickness, as described in relation to experiencing virtual reality and augmented reality (see Background: Health Effects: More screen sensitivity literature).

Comparison of LED symptoms to symptoms of other conditions:

1. Eyestrain/asthenopia and Computer Vision Syndrome

"Eyestrain" is considered to be synonymous with "asthenopia" (Greek for "eye weakness"), the diagnosis in the International Classification of Diseases (ICD-10-CM H53.14) for the collection of symptoms that includes eyestrain, eye fatigue, headache, burning eyes, eye irritation, eye pain, eye ache, eye discomfort, sore eyes, dry eyes, tearing, itching eyes, blurry vision, double-vision, photophobia, and a feeling of having something foreign in one's eye. Eyestrain has been recognized by optometrists as a common phenomenon among computer users - also described in the literature as users of "visual display units" or "visual display terminals" - for decades (Dain et al., 1988; Sheedy, 1992). However, Wilkins (1995) suggests that in many cases "eye strain" would be better described as "brain strain" because there seems to be a role for neurological processing in the brain in creating pain and other symptoms.

Sheedy et al. (2003), in a study of volunteers with normal vision taxed with extreme reading conditions, suggest that there are at least two distinct patterns of asthenopia (eyestrain) symptoms that are associated with different underlying causes. These are an external symptom factor (ESF) pattern (dry eyes, burning eyes, and eye irritation localized at the front and bottom of the eye) and an internal symptom factor (ISF) pattern (eyestrain and eye ache behind the eye and headache). ESF symptoms were associated with testing conditions that caused lower rates of blinking, including holding the eyelids open. The ISF symptom pattern was associated with very close reading distance, slightly out-of-focus reading, or a condition mimicking astigmatism, leading Sheedy et al. to speculate that ISF symptoms may result from difficulties with the coordination of near vision. This coordination, the accommodation-convergence reflex, is the coordinated inward turning of the eyes, increasing curvature of the lens due to constriction of the ciliary muscle, and the pupil constricting when viewing something at a close distance (see Background: Health Effects: Normal eye responses to light).

Asthenopia/eyestrain symptoms are a major subset of the symptoms described as Computer Vision Syndrome (CVS). The American Optometric Association lists common symptoms of CVS as eyes being tired from screen overuse (eyestrain), headaches, blurred vision, dry eyes, and shoulder pain. They suggest possible causes are the overuse of screens, insufficient coordination between the two eyes and/or the brain, screen glare, improper viewing distance, and poor posture. "Computer Vision Syndrome" first appears in PubMed in 1993 and Thompson (1998) is cited in later papers addressing CVS as a review of computer screen-associated vision problems (Rosenfield, 2011). The American Optometric Association website is cited as the main reference describing CVS. The literature on CVS (reviewed in Sheppard & Wolffsohn, 2018) tends to consider how ergonomic changes or interventions to improve eye health, to rest they eyes, or to correct vision with various types of lenses might be helpful to patients. There is also a consideration of whether blue-light blocking lenses might be helpful, with the limited studies suggesting they are not helpful, except for a study indicating that some wraparound-style lenses might be helpful because they reduce tear evaporation rather than because they block blue light (reviewed in Sheppard & Wolffsohn, 2018; Lawrenson et al., 2017; Vagge et al., 2021; see Background: Health effects of blue light). Optometrists' studies of CVS do not seem to include an examination of the role of screen flicker or ambient light flicker in contributing to patients' symptoms.

Most of the people in the survey have at least some symptoms consistent with eyestrain or CVS. In particular, 9/11 people report pain in their eye muscles or when focusing their eyes (Fig. 20). Three of these people and an additional 1 person report pain associated with eye movement only when the headache caused by their LED symptoms is most severe. However, eyestrain and CVS do not explain the range of symptoms of survey respondents, which span several other symptom categories, including various kinds of eye and head pain not associated with eye movement or focusing, short-term memory impairment, sleep abnormalities, spatial disorientation, nausea, and additional symptoms consistent with sensitization of the central nervous system (see below). Eyestrain or CVS also do not explain why 10/11 people have symptoms caused by ambient LED lights, which are unrelated to using the eyes for screen use or reading, thus suggesting that the cause of LED symptoms might not be fully encompassed by the hypothesized causes of eyestrain or CVS described above. Also, since survey responses indicate that their LED symptoms seem to be a relatively recent phenomenon associated with the introduction of LED lights and LED screens, the 1990s-era characterizations of computer-associated asthenopia (eyestrain) and CVS may require updating to reflect symptoms associated with modern screen use.

There is one survey respondent (timestamp 7_2_2021_19_53) who reported that prismatic lenses help "a lot" to prevent the symptoms they experience from LED screens, suggesting a possible eye convergence insufficiency or strabismus (Fig 107; see further discussion below), and who reports eye pain symptoms consistent with the use of eye muscles. However, even this individual's symptoms do not completely fit with a traditional description of eyestrain, as the individual does not report several of the above common symptoms of eyestrain, does not report trouble concentrating when reading, and additionally reports seizures and sleep abnormalities.

2. Dry eye disease

Estimates of the prevalence of dry eye disease (reviewed in Diel et al. (2021)) vary from 5% to 50% of the population, with greater incidence in women. Symptoms of dry eye disease include eye dryness, eye pain, eye discomfort, eye stinging, eye burning, a sensation that a foreign body is in the eye, eye redness, watery eyes, blurred vision, eye fatigue, and photophobia (sensitivity to light). Eye pain or discomfort may occur without any external stimulus, or might be triggered by an ordinary level of light or by wind. Dry eye disease may cause eyestrain.

Patients experiencing the symptoms of dry eye disease sometimes have symptoms that can be diagnosed clinically, including decreased tear production, increased tear breakup, corneal staining, or inflammation at the eye surface. However, sometimes there are not any of these clinical signs and dry eye patients experience the sensations associated with dry eye disease without there being any apparent eye surface abnormality. Research suggests that symptoms experienced by these individuals may result from neurological signaling through the central and peripheral nervous systems initiated by photophobia (Diel et al., 2021, and discussed further below).

Dry eye is one of the most commonly reported symptoms in the LED sensitivity survey with 10/11 people reporting dry eye, all reporting some form of photophobia, and most reporting that they are more sensitive to LED lights/screens when they are experiencing LED symptoms. However, dry eye disease does not explain the range of symptoms of survey respondents, which span several other symptom categories, including various kinds of eye and head pain not directly associated with dry eyes, short-term memory impairment, sleep abnormalities, spatial disorientation, and nausea. The report of other symptoms consistent with sensitization of the nervous system (see below) among those with LED symptoms suggests the possibility that sensitization of the nervous system could create the wide range of symptoms in response to LED light or screens in survey respondents. While it's possible that some survey respondents might have eyes that are actually dry, nervous system sensitization might create the sensation of eye dryness in some, as described above for dry dye disease itself. It's also possible that nervous system sensitization or neuroinflammation could create the pain in eye muscles associated with eyestrain even if the eyes are not overused.

Figures 20, 21, 24, and 26 show frequencies of eye-associated symptoms that include symptoms of eyestrain or dry eye disease.

3. Migraine and Tension-Type Headaches

Migraine and tension-type headache (TTH) are two of the most common classes of headache (described in the International Classification of Headache Disorders, ICHD). Migraine is an often unilateral, pulsatile, moderate to severe headache lasting 4-72 hours, often with photophobia (sensitivity to light), phonophobia (sensitivity to sound), nausea, vomiting, and symptoms aggravated by movement. The photophobia experienced during a migraine is sensitivity to light brightness. Light of normal brightness can cause pain or cause the worsening of migraine symptoms so that being in the dark tends to make the patient more comfortable during a migraine. TTH feels like a band is pressing around the head. It is usually a bilateral, mild to moderate headache lasting 30 minutes to 7 days that does not intensify with exercise, does not include nausea or vomiting, and may include either photophobia or phonophobia, but not both.

Additionally, a subset of migraine patients may experience vestibular migraine, in which vertigo, disorientation, or postural instability may occur at the same time or at different times than their other migraine symptoms (ICHD, reviewed in Nowaczewska, 2020).

While many individuals with LED sensitivity experience symptoms that overlap with some of the features of either migraine or TTH, including headache (9/11), nausea (4/11, but only continues when the lights/screen are turned off for 1/11 people), spatial disorientation (2/11), photophobia (10/11), and phonophobia (3/11), there is not a very good match between LED symptoms and previously-described headache syndromes. The above commonalities do not fully encompass LED sensitivity symptoms and there are some features of migraine or TTH that are not well represented among individuals' LED symptoms. Additional LED symptoms include the very long duration and very high frequency of symptoms for some (Figs. 16, 19), sensitivity specifically to artificial lights/screens, memory and concentration problems, sleep abnormalities, and pain concentrated in or near eyes or symptoms similar to eyestrain or dry eye disease. Although vomiting and the exacerbation of symptoms by exercise are frequent migraine symptoms, no survey respondents report vomiting as a symptom of LED sensitivity and only one reports that their symptoms are worsened by exercise.

Additionally, multiple survey respondents describe how their headaches feel differently than how migraines or TTH feel, While migraine tends to have throbbing in time with the heartbeat and TTH tends to feel like a tight band pressing around the head, 6/9 survey respondents describe their LED headaches as a feeling of swelling or pressure inside the head rather than choosing either the migraine or TTH headache descriptors (Fig. 22).

Given the poor overlap of LED sensitivity symptoms with either migraine or TTH, and also given that LED sensitivity symptoms seem to be a relatively new phenomenon, it is very possible that LED lights and screens trigger headaches of a type not yet clearly defined in the ICHD. The very high rate of headaches in Beijing IT workers in 2018, which were difficult to diagnose due to unusually high rates of photophobia (Li et al., 2020; see section V.3 of that paper), could be explained if the range of headaches includes a new class of headache resulting from LED light and/or screen sensitivity (also discussed above and in Background: Health effects from ≥100 Hz flicker).

4. Post-concussion syndrome

Concussion is considered to be a mild form of traumatic brain injury (mTBI). Symptoms of post-concussion syndrome (PCS) include, cognitive deficits, memory and concentration problems, headache, nausea, dizziness, balance problems, sleep abnormalities, photophobia, phonophobia, tinnitus, anxiety, irritability, and depression (Mayer et al., 2017). The disease process in concussion has at least two significant phases. First, there is the damage to the brain as a result of the original physical trauma. Second, there is inflammation and abnormal healing in cells and tissues in the brain, that can lead to further neuron and tissue damage and abnormal signaling in the brain, causing a variety of neurological symptoms. Such neurorinflammation and abnormal signaling might last up to years in a subset of patients with post-concussion syndrome (reviewed in Simon et al., 2017).

Headache is the most common symptom in both the acute and chronic phases following a concussion. Post-traumatic headache (PTH) is headache that can develop in the weeks following a concussion and is more common in women. It may resemble migraine, TTH, or both (reviewed in Mares et al., 2019). Additionally, the CDC reports that patients may report either "headache or 'pressure' in the head" and the Sport Concussion Assessment Tool (SCAT5) diagnostic tool for sports-related concussion includes both "headache" and "pressure in head" among the symptoms for which patients are asked to indicate the severity.

Photophobia, especially sensitivity to artificial lights and screens, is the second most common symptom in post-concussion syndrome and it may aggravate headache symptoms. PTH and photophobia usually occur together (reviewed in Mares et al., 2019).

Assessments of cognitive deficits are common tests in the diagnosis of concussion (Tator, 2013), including in the SCAT5. Cognitive deficits tend to improve within about 3 months of a concussion, but occasionally persist for longer periods of time.

In the acute phase in the first week after a concussion, patients tend to sleep more than usual (hypersomnia). However, in the post-acute phase, concussion patients report insomnia (trouble falling asleep or early morning awakenings) and daytime fatigue that can occur for weeks to years after the acute injury phase. Mosti et al. (2016) review concussion-related sleep abnormalities and also discuss various sleep surveys that clinicians might use to evaluate sleep.

Individuals with a history of concussions are at particular risk of developing dementia or multiple sclerosis (Montgomery et al., 2017, also described in the Harvard Health Blog), neurodegeneration, or chronic traumatic encephalopathy (reviewed in Simon et al., 2017), although it isn't yet known what the relative contributions of the primary physical trauma and the secondary neuroinflammatory signaling are to these disease processes.

Although none of the current LED survey respondents report a history of concussion, it is intriguing that many of the symptoms described by survey respondents overlap with concussion symptoms, particularly the nature of headache pain (including the description of "pressure" in the head), spatial disorientation symptoms, sleep abnormalities that include both hypersomnia and insomnia, fatigue, concentration/short-term memory difficulties, photophobia (especially artificial light and screen sensitivity), phonophobia, tinnitus, anxiety, irritability, and depression. The more common post-concussion symptoms also occur frequently in respondents to the LED Sensitivity Survey. 10/11 survey respondents report "pressure" near their eye(s) or in their head, similar to descriptions of PTH pain. All survey respondents report pressure and/or headache that they attribute to LEDs. The descriptions of these LED headaches vary from person to person, similar to how the characteristics of PTH vary from person to person. All are sensitive to LED lights and/or LED screens and almost all report more sensitivity when experiencing LED symptoms. 6/11 have sleep abnormalities, 6/11 have concentration problems, 5/11 have short-term memory problems, and 4/11 experience spatial disorientation. Additional symptoms that may be evidence of sensitization of the nervous system like visual disturbances, phonophobia, tinnitus, and allodynia (pain from innocuous touch) are also shared by both people experiencing LED symptoms and patients with PCS. The chronic and recurring nature of the symptoms is another commonality between LED symptoms and PCS. A main area where LED symptoms differ from PCS symptoms is in the high incidence of dry eye and various eyestrain symptoms in people with LED symptoms. However, it's also possible for these symptoms to occur due to sensitization of the nervous system (see below).

The symptom overlap between concussion and LED symptoms suggests the possibility that some of the same nervous system and/or neuroinflammatory pathways that are triggered in concussion might also be triggered by LED light in some LED-sensitive people.

Neuroinflammation is known to occur in the brain following a concussion and has been suggested to play a role in mediating long-term symptoms (reviewed in Simon et al., 2017). The ability of ≥100 Hz flicker to cause neuroinflammation has not been tested to my knowledge, but visible 40 Hz flicker has been shown to cause neuroinflammation in the brains of mice (Iaccarino, 2016, Garza et al., 2020 and reviewed in Thomson, 2018). If the flicker of LED lights has the potential to initiate neuroinflammation in sensitive individuals, this could potentially explain the long-term LED symptoms that mimic concussion symptoms.

While there is substantial overlap between LED symptoms and concussion symptoms, respondents to the LED sensitivity survey currently do not include people who have previously had a concussion or other brain injury, suggesting that having had a concussion is not a prerequisite for experiencing LED symptoms. However, a reason why those with concussion are sensitive to artificial lights and screens might be because flickering LED light can trigger the same kind of symptoms as those that are triggered in concussion. The LED lights and screens could be restarting post-concussion symptoms and may be increasing neuroinflammation and nervous system sensitization (see below). This is consistent with the observation that PTH and photophobia usually occur together in post-concussion syndrome (reviewed in Mares et al., 2019).

5. Flicker vertigo

"Flicker vertigo" is terminology used in aviation to describe symptoms caused by visibly flashing lights (rotating beacons or strobe lights) or by the flicker of sunlight observed through a rotating propeller, especially at frequencies between 4 Hz and 20 Hz (reviewed by Clarence E. Rash). Symptoms include nausea, dizziness, confusion, panic, vomiting, spatial disorientation, and, rarely, seizures and loss of consciousness. Symptoms are sometimes barely-noticeable, vague discomfort. Sometimes symptoms stop when the flicker stops and sometimes persist after the flicker ends.

Flicker vertigo is most associated with flicker frequencies that are slower than those produced by LED lights, but could sometimes occur on screens and that are common in the environment, such as lights on emergency vehicles and flashing bicycle lights. When asked specifically about their response to visibly flashing lights, 7/11 respondents report sensitivity to visibly flashing lights with symptoms that include pain, nausea, spatial disorientation, and, in one individual, seizures (see Figs 11-12).

The vomiting, panic, seizures, and loss of consciousness described for flicker vertigo are generally more extreme symptoms than those reported in response to LED lights and screens in the LED Sensitivity Survey, except that one respondent reports seizures. However, survey respondents report other symptoms frequently, including nausea, spatial disorientation, and concentration and short term memory problems that might be similar to "confusion." While survey respondents do not report panic, many report anxiety. There are many other types of symptoms reported in the LED Sensitivity Survey that are not described by flicker vertigo, including dry eye, eyestrain, headache, and sleep abnormalities. It is not surprising that there is not complete symptom overlap between flicker vertigo and LED sensitivity since flicker vertigo involves slower flicker for briefer periods of time, while LED sensitivity generally involves faster flicker with exposures that generally last for longer periods of time.

6. Cybersickness

In the scientific literature, cybersickness is associated with the use of virtual reality or augmented reality headsets (see Background: Health Effects: More screen sensitivity literature). Symptoms include "nausea, disorientation, oculomotor disturbances, drowsiness (a.k.a sopite syndrome) and other discomforts" (Stanney et al., 2020). Symptoms that may linger long after the VR or AR experience "can compromise postural stability, hand-eye coordination, visual functioning, and general well-being" (Stanney et al., 2020). "Cybersickness" has also been suggested in anecdotal reports (see Anecdotal reports of LED sensitivity) as the explanation for adverse health effects experienced by users of normal screens like cell phones and tablets.

The descriptions of the symptoms of cybersickness match many of the symptoms reported in the LED Sensitivity Survey, including nausea, spatial disorientation, and falling asleep early or at inappropriate times. The various kinds of visual disturbances and difficulty using the eyes reported by many in the LED Sensitivity Survey could also fit with the visual functioning and oculomotor disturbances of cybersickness. The lingering spatial disorientation reported by 2/11 survey respondents might fit with the description of lingering compromised postural stability in cybersickness. Cybersickness best describes the symptoms in the LED Sensitivity Survey that are most related to nausea and spacial disorientation. However, descriptions of cybersickness do not encompass the full range of LED symptoms, which also include dry eye, eyestrain, headache, concentration and short-term memory problems, and insomnia.

All photophobia is not the same: Sensitivity to light brightness vs. sensitivity to flicker

Migraine, dry eye, and concussion have been suggested to share similar nervous system signaling because of their partial sharing of symptoms, especially photophobia (Diel et al., 2021). The collective symptoms of these conditions have a large amount of overlap with LED symptoms, which also involves photophobia. While all of these conditions can involve photophobia, all photophobia is not the same.

Photophobia, feeling pain or an exacerbation of symptoms in response to light, may be a symptom of migraine, dry eye, concussion (mild traumatic brain injury), tension-type headache, blepharospasm (involuntary eye blinking and twitching), and many other conditions (listed in Katz and Digre, 2016).

Noseda & Burstein (2011) carefully distinguish between three distinct types of "photophobia" experienced by migraine patients: (1) exacerbation of headache pain by light of ordinary brightness in ~80% of migraine patients, (2) perception of an ordinary level of light as too bright by some not well-defined fraction of migraine patients, and (3) light of ordinary brightness causing eye pain in some not well-defined fraction of migraine patients. Migraine patients with photophobia tend to prefer to be in the dark during a migraine.

While migraine photophobia tends to be sensitivity to light brightness, this does not necessarily seem to be the case for at least some other kinds of photophobia. In a review article that generalizes trends seen by ophthamologists treating patients with photophobia, Katz and Digre (2016) say that patients with photophobia often report being particularly sensitive to computer screens or to artificial light, with "perspicacious" patients recognizing that their sensitivity is specifically to nonincandescent artificial indoor light. At the time of their review article, that artificial nonincandescent light could presumably be either fluorescent or LED light. The reference given for this statement, other than presumably their clinical experience, is Wilkins and Wilkinson (1991) which focuses on the health effects of flicker from fluorescent lights (see Background: Health effects of blue light which discusses this paper's focus on the blue light flicker of a subset of fluorescent lights).

Katz and Digre (2016) do not indicate whether there are particular photophobia conditions that are more likely to be characterized by sensitivity to artificial light and/or screens. However, other sources suggest that artificial light and screen sensitivity may often characterize the photophobia in post concussion syndrome (Truong et al., 2014; reviewed in Mares et al., 2019). Concussion patients are advised to avoid screen time because it can worsen concussion symptoms (SCAT5, 2017). The focus on artificial lights and screen use as problematic for concussion patients suggests that at least part of the photophobia in concussion might be associated with light flicker. In fact, Mansur et al. (2018) suggest that the computer screen sensitivity of concussion patients may be associated with the flicker created by the refreshing of computer screens. They show some benefit to concussion patients of using a prototype eInk screen that only refreshes when the screen content changes. (Their choice to use 8-12 Hz frames per second during necessary refreshing is curious since that would presumably create quite visible flicker).

It is less clear what aspect of light is problematic for patients with photophobia due to dry eye disease. The Ocular Surface Disease Index Survey designed to assess dry eye disease asks patients if they are sensitive to light, but does not address the kind of light. A survey by Kaido et al. (2014) shows a higher rate of people with dry eye disease agreeing with a question asking if they are sensitive to bright light (21%) compared to normal control individuals (4%), but this survey does not include any questions that might be applicable to light flicker. Also, while screen use has been hypothesized to be a risk factor for dry eye disease, this has not been definitively shown, partly due to inconsistencies in study designs (meta-analysis in Courtin et al., 2016).

Dry eye disease is found more frequently in migraine patients and in concussion patients than in normal controls (Diel et al., 2021). While Diel et al. propose that there may be overlap of the underlying mechanisms of disease in these disorders, it isn't clear whether there is overlap in the particular type of photophobia if patients experience more than one condition.

Wilkins et al. (2021) discuss how "photophobia" could mean sensitivity to light brightness, sensitivity to light flicker, sensitivity to pattern, or sensitivity to color.

It's possible that people may experience more than one type of photophobia, either at different times or concurrently, depending on stimuli in the environment, underlying nervous system signaling, and on any underlying inflammatory state. For example, of the 8 people in the survey who report another kind of headache in addition to their LED symptoms, 5 of them report that the types of light that are bothersome during their LED symptoms differ from the types of light that are bothersome during their other headaches.

The following table reports photophobia-associated conditions that are listed in the medical histories of those reporting LED sensitivity:

Photophobia-associated conditions reported in the medical histories of survey respondents:

While the relevance of any of the above conditions to LED sensitivity is unknown, it is intriguing that so many (8/11) survey respondents report a medical history of one or more conditions that is at least sometimes associated with photophobia and 5/8 described photophobia as a symptom of their other non-LED headaches. It would be interesting to study whether there are shared neurological processes/conditions that underlie both LED sensitivity and the above conditions or whether the pathology of the above conditions creates conditions in the nervous system that predispose individuals to LED sensitivity. One hypothesis is that sensitization of the trigeminal sensory system by the above conditions might predispose individuals to be sensitive to LED flicker.

What is the trigeminal sensory system and what is its role in photophobia syndromes?

Altered signaling through neurons of the trigeminal sensory system occurs in dry eye disease, migraine, and concussion. This portion of the nervous system is responsible for sensing and transmitting pain, touch, and temperature in the face and head.

What is a neuron?

A neuron is a nerve cell. One side of a neuron receives signals and the other end sends signals. Extending from the cell body of the neuron are dendrites that initiate signaling in response to stimuli and axons that continue to send those signals toward the far end of the neuron, the axon terminus. A signal may then be transferred across the small gap between neurons, by chemicals diffusing from an axon terminus of one neuron to a dendrite of the next neuron.

What are retinal ganglion cells?

Typical retinal ganglion cells are cells in the eye that do not detect light themselves, but that are activated by light-sensing retinal rod and cone cells. The retinal ganglion cells are largely responsible for passing along signals from rods and cones via the optic nerve for image-forming vision.

Intrinsically photosensitive retinal ganglion cells (ipRGCs) contain the photopigment melanopsin, so they can sense light, but they are relatively few in number and scattered over the retina. They have a limited role in image-forming vision and tend to largely have a role in sensing light brightness and enhancing image contrast. In addition to being directly activated by light, ipRGCs are also activated by signals from rods and cones via retinal ganglion cells.

The trigeminal sensory system consists of:

(1) "First order" pain-sensing, touch-sensing, and temperature-sensing neurons in the face/head.

The ends of the neurons that sense pain, touch, or temperature are located in peripheral locations in the face or head, the neurons then pass through the trigeminal ganglion near the temple (where the cell bodies of these neurons are located) and then terminate in the spinal trigeminal nucleus. Together, these sensory neurons are called the "trigeminal nerve," or cranial nerve V, which has three branches responsible for sensing pain, touch, and temperature in different parts of the face/head: the ophthalmic branch (top of face/head; eye and upper eyelid and above), the maxillary branch (middle of face; from lower eyelid to upper lip), and the mandibular branch (lower part of face; lower lip and below). There are two trigeminal nerves; one on the left side of the head, and one on the right side of the head.

First order sensory trigeminal neurons within the ophthalmic branch of the trigeminal nerve that may play roles in photophobia conditions are:

a. Trigeminal neurons sensing pain in eye blood vessels:

During migraine, these neurons play a role in the perception of ordinary light brightness as pain in the eyes (see explanation below).

b. Trigeminal neurons containing melanopsin that begin in the iris, cornea, and choroid of the eye (Matynia et al., 2016):

Like any first-order trigeminal neurons, these neurons that begin in the eye are thought to sense pain, touch, or temperature, but they can also sense light since they contain the light-sensing photopigment melanopsin. These neurons may create a feeling of pain in the eye in response to light.

c. Trigeminal neurons sensing pain on the surface of the cornea:

These neurons may be significant in dry eye disease

d. Trigeminal neurons sensing light brightness in the retina (Dolgonos et al., 2011)

These neurons are thought to enhance the trigeminal blink reflex.

e. Trigeminal neurons sensing pain in the the dura of the meninges (the tissue covering the outer surface of the brain):

These neurons play a role in creating the head pain in migraine headache.

(2a) "Second order" neurons that receive signals from first order neurons of the trigeminal nerve in the spinal trigeminal nucleus and then transmit signals to third order neurons in the posterior thalamus of the brain.

The thalamus is located in the interior of the brain, just above the brainstem. A main function of the thalamus is to relay sensory signals originating in the peripheral nervous system to the brain's cerebral cortex in the central nervous system.

(3a) Third order neurons that receive signals from the termini of second order neurons in the posterior thalamus and then send signals to many different sensory processing parts of the cerebral cortex of the brain including the following (functions in parentheses):

a. Visual cortex (visual perception)

b. Retrosplenial cortex (memory, cognition, spatial navigation, planning for the future)

c. Parietal association cortex, also called the posterior parietal cortex (mapping of environmental spatial coordinates relative to the body; planning and coordinating movement relative to environmental space)

d. Auditory cortex (sound perception)

e. Somatosensory cortex (pain, touch, and temperature perception)

f. Motor cortex (initiation of movement and suppression of movement)

g. Olfactory cortex (odor perception)

(2b and 3b) The hypothalamus: There is also a trigeminohypothalamic tract in which "second order" neurons receive signals from first order neurons of the trigeminal nerve in the spinal trigeminal nucleus and then transmit signals to the hypothalamus of the brain. The hypothalamus is located just below the thalamus. Signaling from the hypothalamus can control many functions including:

a. Circadian rhythm

b. Autonomic functions: nausea, vomiting, dizziness, yawning, tearing

c. Homeostasis: hunger, fatigue, temperature regulation

d. Anxiety, irritability, depression

First order pain-sensing trigeminal neurons originating in the dural meninges convey pain signals during migraine terminate in the spinal trigeminal nucleus. There, second order neurons convey the signal to the hypothalamus, which in turn creates signaling that may result in loss of appetite, fatigue, anxiety, irritability, depression, nausea, vomiting, yawning, tearing, chills, or sweating (reviewed in May and Burstein, 2019).

The hypothalamus also receives direct input from the retina, including from retinal ganglion cells, and light can increase hypothalamus-mediated symptoms during migraine (Noseda et al., 2017, reviewed in May and Burstein, 2019).

Sensitization of the trigeminal sensory system

Some symptoms of migraine, dry eye, and concussion suggest that abnormal sustained signaling through the trigeminal sensory pathway may sensitize and cause inflammation in the trigeminal pathway, including in the trigeminal ganglion, spinal trigeminal nucleus, and posterior thalamus.

Sensitization: Sensitization involves permanent structural and molecular changes to neurons that creates abnormal signaling. Signaling could be enhanced along an existing pathway or signaling could spread to nearby neurons.

Neuroinflammation: In general, symptoms of localized inflammation are redness, heat and swelling of the affected tissue. This happens because release of immune system signaling molecules causes blood vessels to dilate, allowing greater flow of fluid and immune blood cells out of the blood vessels into the surrounding tissue. The increased fluid in the tissue creates swelling. Greater blood flow to the tissue creates increased heat and redness. Similarly to other kinds of inflammation, neuroinflammation involves dilation of blood vessels, flow of fluid into surrounding tissue, and activation of immune system signaling (reviewed in Ashina et al., 2019). During neuroinflammation, cells release the protein calcitonin gene-related peptide (CGRP). CGRP causes dilation of blood vessels in the brain and activates immune system signaling (Ashina et al., 2019). CGRP also causes sensitization of neurons. CGRP is produced in the dural meninges during migraine. CGRP is also produced in the trigeminal ganglion and spinal trigeminal nucleus where it leads to sensitization. The hormone melatonin, which is released in the dark by the pineal gland and regulates sleep, inhibits CGRP synthesis (Peres et al., 2019).

What causes eye pain during migraine light brightness photophobia?

Melanopsin is a light-sensing photopigment. When a person experiences migraine light brightness photophobia, light is thought to activate the retina's melanopsin-containing intrinsically photosensitive retinal ganglion cells (ipRGCs) which signal via the midbrain to cause dilation of eye blood vessels (reviewed in Noseda & Burstein, 2011, Noseda et al., 2019). First order pain-sensing trigeminal neurons sense the dilation of eye blood vessels and signal through the trigeminal ganglion to the spinal trigeminal nucleus. There, second order trigeminal neurons receive the pain signal and send it to the thalamus where third-order trigeminal neurons receive the signal and send it to the somatosensory cortex. Pain is perceived in the somatosensory cortex. Thus, light brightness creates a feeling of pain in the eye.

While ipRGCs typically sense light and initiate signaling in response to that light, it isn't typical for normal levels of light brightness to be perceived as painful. CGRP production within the trigeminal signaling pathway during migraine is thought to create conditions that make the normally innocuous light brightness be perceived as painful.

How does light brightness increase migraine headache pain?

While migraine can have many symptoms in addition to headache, the headache pain itself is typically localized in the dural meninges, the tissue covering the outer surface of the brain. During migraine, CGRP is produced in the dural meninges, contributing to causing the headache pain. Pain-sensing first order trigeminal neurons send the pain signal from the dural meninges via the trigeminal ganglion to the spinal trigeminal nucleus. There, second order trigeminal neurons send the pain signal to the thalamus where third order neurons relay the pain signal to the somatosensory cortex.

In the posterior thalamus during migraine, the third order neurons that receive trigeminal sensory pathway pain signals originating in the dura of the meninges (the location of the headache) also receive signals directly from axons of the eye's retinal ganglion cells, with about 80% of those being ipRGCs. From the thalamus, dura- and light-sensitive third order neurons send signals to the somatosensory cortex where pain is perceived. Since pain from the dura and light activate the same third order neurons, light is thought to intensify the perception of dural headache pain (reviewed in Noseda & Burstein, 2011, Noseda et al., 2019). This pathway is thought to be the means by which headache pain is exacerbated by light in the 80% of migraine patients with this type of photophobia during their migraine.

How does light impact dry eye symptoms?

In dry eye disease, first order trigeminal neurons perceive pain at the corneal surface, sometimes due to eye dryness or some other ocular issue like an infection. These first order neurons extend through the trigeminal ganglion to the spinal trigeminal nucleus. Second order trigeminal neurons then transmit the signal to the thalamus where third order neurons transmit the pain signal to the cerebral cortex.

In dry eye disease, there is increased CGRP within the trigeminal sensory system that is thought to lead to sensitization and increased pain perception. Many patients can experience the pain of dry eye disease without actually having dry eyes.

For people who experience photophobia as part of their dry eye disease, signaling via ipRGCs that sense light is thought to intensify signaling through the trigeminal sensory system.

How does sensitization create abnormal signaling in the cerebral cortex?

In addition to enhancing signaling along established neural pathways, sensitization can also create new signals to atypical locations in the brain.

The thalamus is the relay center of the brain. In the thalamus, sensitization may cause signals from second order trigeminal neurons to be able to activate more third order trigeminal neurons than they would normally activate, creating atypical signaling to the cerebral cortex. Sensitization of other areas of the trigeminal sensory system such as the trigeminal ganglion or spinal trigeminal nucleus can also lead to abnormal signaling. Essentially, sensitization and neuroinflammation along the trigeminal sensory pathway makes signaling along that pathway somewhat leaky, creating unusual cross-talk among nearby neurons.

This abnormal signaling to the cerebral cortex can impair normal cortical function, create abnormal perceived sensations, or creating exaggerated responses (reviewed in Burstein et al., 2019). Examples of the symptoms that may result from such abnormal signaling to the cerebral cortex are listed below in bold.

a. Visual cortex (various visual disturbances)

b. Retrosplenial cortex (impaired cognitive function, decreased short term memory)

c. Parietal association cortex (decreased attention in spatial mapping, spatial disorientation)

d. Auditory cortex (phonophobia - abnormal sensitivity to sound)

e. Somatosensory cortex (photophobia - ordinary light brightness is painful or exacerbates other symptoms; phonophobia - ordinary level of sound is painful; increased perception of eye dryness/pain even if eyes aren't actually dry in dry eye disease; allodynia - skin painful if touched in a way that would normally not hurt; hyperalgesia - increased pain sensed from typically only slightly painful stimuli like wind in the eyes)

f. Motor cortex (increased muscle weakness, decreased motor coordination)

g. Olfactory cortex (osmophobia - abnormal sensitivity to smells)

Thus, sensitization causing abnormal signaling within the cerebral cortex explains why unusual-seeming symptoms like allodynia, hyperalgesia, short-term memory deficit, spatial disorientation, visual disturbances, and tinnitus can be present in conditions like migraine or concussion, where signaling occurs via the trigeminal sensory system. In fact, allodynia is considered to be a marker of central nervous system sensitization. Some of these kinds of symptoms, like hyperalgesia, may also be present in dry eye disease.

What roles do sensitization and neuroinflammation play in creating post-concussion syndrome?

The physical injury to the brain during a concussion initiates a process of neuroinflammation in the brain that includes the production of CGRP, which can create further inflammation, pain sensitivity, and sensitization of the nervous system. In particular, CGRP-mediated neuroinflammation and sensitization within the trigeminal sensory system are thought to cause post-traumatic headache (Mares et al., 2019). Sensitization of the nervous system could explain many of the other symptoms of post-concussion syndrome as well, including the spatial disorientation, and concentration and short-term memory problems.

Could physiological health effects of LED flicker mimic the nervous system signaling, sensitization, and neuroinflammation present in dry eye disease, migraine, and/or concussion?

We don't know, but this makes sense. The data from the LED Sensitivity Survey are consistent with the hypothesis that LED flicker causes nervous system sensitization and neuroinflammation, resulting in symptoms that mimic those in dry eye disease, migraine, and concussion.

  • All LED survey respondents reporting sensitivity to LED lights and/or screens (11/11) are indicating that they experience a form of photophobia that is specific to artificial lights and/or screens.

  • Most (8/11) report pain in or near the eyes that starts when they look at the lights/screen and that goes away when they stop looking at the lights/screen. Seven of these individuals report that the this includes pain that is not associated with eye muscles or focusing, with the most common description from 6 individuals being "sharp, focused pain in or near the eye that feels like pinching or cutting (not related to eye movement or focusing)."

    • Conjecture: Eye pain that is present only when actually observing the LED lights and/or screens could be analagous to eye pain caused by light brightness during migraine that is present only when actually observing the light. Since the light brightness pain in migraine is caused by light indirectly activating pain-sensing first order trigeminal neurons (see section above), it's possible that light or screen flicker might also be directly or indirectly activating first order trigeminal neurons in the vicinity of the eye that then send the pain signal through the rest of the trigeminal sensory pathway so that pain is perceived. Flicker probably activates trigeminal signaling through a different mechanism than light brightness during migraine. However, it's possible that the mechanism could be the same or similar to how artificial lights and screens trigger pain in many patients with post-concussion syndrome, since that sensitivity might also be due to flicker.

  • In dry eye, migraine or concussion, sensitization of the trigeminal sensory system can create altered signaling via the thalamus to the cerebral cortex, causing symptoms that result from enhanced or unusual signaling in the cerebral cortex (see section above). Many of these symptoms are also reported by individuals with LED symptoms, including allodynia (pain from innocuous touch), concentration and short-term memory problems, spatial disorientation, visual disturbances, and tinnitus. In dry eye, migraine or concussion, sensitization can also create altered signaling from the trigeminal sensory system through the hypothalamus, causing a variety of symptoms (see section above). Many of these symptoms are also reported by individuals with LED symptoms, including sleep abnormalities, nausea, loss of appetite, feelings of warmth, anxiety, irritability, and depression.

    • Conjecture: The presence of these symptoms in survey respondents suggests that exposure to LED lights and/ore screens may cause sensitization of the trigeminal signaling system. In dry eye, migraine, and concussion, sensitization is often also associated with neuroinflammation and the release of CGRP. The similarities between the LED symptoms that may result from sensitization and symptoms probably resulting from sensitization in migraine, post-concussion syndrome, or dry eye disease support the hypothesis that there may be some commonalities in how the nervous system or neuoinflammatory signaling pathways play roles in creating these symptoms in all of these conditions.

  • 10/11 individuals report dry eye(s) being caused by LED lights and/or screens

    • Conjecture: When experiencing LED symptoms, the sensation of dry eye could result from enhanced pain signaling from the eye surface through the trigeminal sensory system due to the LED light or screen. A similar phenomenon happens in dry eye disease with photophobia, during which light initiates signaling via ipRGCs that enhances the trigeminal pain signaling originating at the surface of the eye. In dry eye disease, this can make people's eyes feel dry even if their eyes are not actually dry. It's possible that light or screen flicker might initiate signaling that similarly enhances transmission of pain signals from the eye surface. In some cases, people's eyes might be somewhat dry to begin with. In some cases, people whose eyes are not actually dry may report experiencing the sensation of having dry eyes if the flicker somehow enhances the feeling of pain coming from the eye surface.

  • 9/11 individuals report pain in eye muscles or pain when focusing as typical symptoms, consistent with the experience of eyestrain. All 9 of these individuals also report dry eye. Three of these people and an additional 1 person report pain associated with eye movement only when the headache caused by their LED symptoms is most severe, described as: "It becomes painful to move eyes from side-to-side in light that is not normally painful, in the dark, and when focused at near or far distances. Keeping the eyes stationary is not painful."

    • Conjecture: There are multiple possibilities for how these symptoms might be created: (1) There might be signaling analogous to how light intensifies migraine headache pain. For example, it's possible that trigeminal neurons involved in signaling pain due to flicker might terminate on the same neurons that transmit pain from eye muscles, thus creating the sensation of eye muscle pain. (2) It's also possible that neuroinflammation in or around the eye might lead to the pain associated with moving or focusing the eyes. The neuroinflammation could result from persistent signaling through the trigeminal sensory system initiated by the photophobia pain describe above. This possibility might explain why some report this feeling of eyestrain only when their LED headache is most severe.

  • 9/11 individuals describe feeling pressure in or near the eye(s). 6/9 individuals describe the headaches they attribute to LEDs as "pressure that feels like there's swelling in or around the eye or deeper inside the head - the pressure is continuous and comes with constant pain."

    • Conjecture: "Pressure in [the] head" is a common descriptor for how some kinds of post-traumatic headache after a concussion feels (CDC, SCAT5). The description of pressure inside the head during LED symptoms, especially the feeling of swelling, differs from how primary headaches (migraine or tension-type headache) are described in diagnostic materials (ICHD). Could the feeling of pressure in the head during LED symptoms be associated with neuroinflammation? How similar is it to the feeling of pressure in PTH and is there any similarity in the cause of the symptoms?

  • 9/11 people report headaches that they attribute to LED lights and/or screens. Different people describe their LED headache pain differently and the headache descriptions do not tend to clearly match descriptions of primary headaches (migraine or tension-type headache) in diagnostic materials (ICHD). However, that variability in headache descriptions for different people and descriptions of "pressure" are typical of post-concussion syndrome. 9/11 people report having had LED symptoms on 15 or more days in the last 30 days, with 7/11 people experiencing symptoms every day (30/30 days). The chronic or frequently-recurring nature of LED symptoms is not typical of how most people with migraine or tension-type headache experience those headaches. However, it is similar to the chronic nature of post-concussion symptoms. Many other LED symptoms are also typical post-concussion symptoms, including the sensitivity specifically to artificial lights or screens, sleep disturbances, concentration problems, short-term memory problems, nausea and spatial disorientation.

    • Conjecture: The similarity between many LED symptoms and post-concussion syndrome symptoms suggests the possibility that there could be some similarities in the underlying signaling in these two conditions. That signaling might involve nervous system sensitization and/or neuroinflammation. This suggests that for people with LED sensitivity, the LED lights and/or screens might somehow trigger processes in the brain that mimic processes that result from mild traumatic brain injury. While much more research would be needed to evaluate the possibility that some of these processes could be the same, until that research is done the following could be prudent:

      • While those with post-concussion symptoms are already advised to avoid screens because they aggravate PCS symptoms, it may be even more important than was previously known for people with PCS to avoid screens and artificial lights with flicker since these might have the ability to cause PCS-like symptoms on their own. The concussion might alter people's sensitivity to screen or light flicker so that levels of flicker that might not normally have bothered people are able to trigger symptoms during PCS and could possibly even extend PCS.

      • It could be in the long-term health interest for those with LED sensitivity to avoid LED light and/or screen flicker. Repeated re-exposure to LED light and/or screen flicker could perpetuate chronic/recurring abnormal signaling and neuroinflammation in the brain similar to that of post-concussion syndrome. While there is no way to know what long-term consequences this might have without further research, it is concerning that recurring concussions are associated with multiple quite serious conditions. Individuals with a history of concussions are at particular risk of developing dementia or multiple sclerosis (Montgomery et al., 2017, also described in the Harvard Health Blog), neurodegeneration, or chronic traumatic encephalopathy (reviewed in Simon et al., 2017), although it isn't yet known what the relative contributions of the primary physical trauma and the secondary neuroinflammatory signaling are to these disease processes. Hopefully there is not reason for concern about chronic LED symptoms leading to similar long-term health consequences - hopefully LED symptoms and concussion are too dissimilar for this to happen. However, since it's possible to use artificial lights that don't cause LED symptoms, such as completely flicker-free LED lights, or for most people, incandescent lights, this possibility is one more reason why lighting choices should be made that protect people's health. Likewise, the computer industry should try to create technology that doesn't trigger screen sensitivity for sensitive people.

Prior nervous system sensitization due to other medical conditions might influence LED sensitivity, and vice versa

Given that nervous system sensitization, especially in the trigeminal sensory system, is a feature of conditions like dry eye disease, migraine, and concussion and that it is possible that similar sensitization may play a role in LED symptoms, it would be interesting to evaluate whether a prior medical history of one of these conditions affects either the likelihood of developing LED sensitivity symptoms or the range of those symptoms in the individual.

As more survey responses are collected, it will be interesting to evaluate whether there is a higher incidence of conditions in the medical histories of respondents that could be associated with nervous system sensitization than would be expected by chance. However, it is already interesting that 8/11 respondents report having other non-LED headaches with 5 of these individuals reporting photophobia as a symptom of their other headaches.

All survey respondents report some form of headache. 6/11 experience both headaches that they attribute to LEDs and other kinds of headaches, 3/11 only report headaches that they attribute to LEDs, and 2/11 only report other kinds of headaches. Respondents report that their LED symptoms occur on significantly more days per month than their other headaches (Fig 105).

Of the 8 out of 11 survey respondents who also experience other types of headaches that they do not associate with LEDs, most indicate that their other headaches started at a younger age than their LED symptoms (Fig 104). For these individuals, it's possible that any nervous system sensitization accompanying their other non-LED headaches might have influenced the development of their LED symptoms.

Conversely, 2 respondents indicate that their other headaches started at the same age or at an older age than their LED symptoms while in their 20s (Fig 104), and the HARDSHIP algorithm classifies the other headaches of both of these individuals as "probable migraine." During their other headaches, both individuals experience throbbing headache on both sides of the head that worsens with exercise and comes with photophobia, that includes sensitivity to both the sun and to artificial light, and phonophobia (sound sensitivity), but not nausea or vomiting. While it isn't known whether their experience with LEDs has also played a role in bringing about other migraine-like headaches, this would be consistent with the hypothesis that nervous system sensitization or neuroinflammation, in this case caused by LEDs, might influence the incidence of conditions that use overlapping nervous system signaling pathways. In further support of this hypothesis, injection of CGRP has been shown to initiate migraine.

Concussion is another condition that can lead to recurring new migraine-like headaches or other kinds of new headaches during post-concussion syndrome. It could be that sensitization and/or inflammation in the nervous system pathways underlying concussion overlap with those involved in migraine, leading to new migraine-like headaches. Perhaps the two individuals in the LED survey with new probable migraine headaches are experiencing a similar phenomenon. This supports the hypothesis that for sensitive individuals, exposure to triggering LED lights or screens may initiate a syndrome similar to post-concussion syndrome.

The above points also raise the possibility that other individuals might be experiencing new migraine-like symptoms that have arisen only since they've acquired LED symptoms, but that they might not be able to distinguish between the common migraine-like symptoms and their other LED symptoms because they don't have the context of prior experience with other headache types and because they (rightly) think of all of their symptoms as associated with LED use.

In further support of the hypothesis that LED symptoms might affect the incidence of other "non-LED" headaches, I've personally noticed that I have experienced more frequent common migraines recently during months-long ongoing LED symptoms (after more than 8 months, the LED symptoms have tapered off a lot, but I am not yet back to my baseline sensitivity to light flicker and still have recurrences of minor LED symptoms on a nearly daily basis). The LED symptoms started following a 3-hour LED light flicker exposure on April 1, 2021. In the following 8 months, I experienced common migraine symptoms identical to those I've experienced since age 13 on about 12 separate days, which is more frequent than my typical couple of days of common migraine per year. During this time, I noticed that the left-side only common migraine typically started several hours to about a day after I had used a screen for too many minutes two days in a row, to the point that the screen use had restarted my right-side only LED headache, head pressure, nausea and disorientation symptoms. I suspect that the nervous system signaling and/or inflammatory signaling happening in response to LED lights may also increase the probability of common migraine onset for me. However, the two headache types still seem somewhat independent in that successful treatment of the common migraine with ibuprofen has no effect on the ongoing LED headache and other LED symptoms (see Self-tests).

It would also be interesting to evaluate whether the number of years of experiencing migraine or other non-LED headaches, dry eye, concussion, or any other condition linked to sensitization influences the incidence of people having LED symptoms.

It could also be interesting to evaluate whether there is any link between the symptoms of individuals' other medical conditions that may involve sensitization and the range of their LED symptoms. For example, there is currently only one individual whose other headaches are classified as migraine (rather than "probable migraine") according to the algorithm for the HARDSHIP questionnaire (Steiner, 2014) and whose symptoms most fully match typical common migraine symptoms. Migraine symptoms for this person are unilateral throbbing headache with light brightness photophobia and symptoms exacerbated by exercise; this person is the only respondent to indicate nausea and vomiting as symptoms of their other headaches. This individual happens to report the longest-lasting, (>2 months) LED sensitivity symptoms following a single LED exposure (although others report never-ending symptoms due to continual re-exposure to LED lights/screens) and is the only person to report LED sensitivity symptoms that include ongoing nausea, loss of appetite and long-lasting insomnia, although others report nausea and insomnia of shorter duration. This individual is one of two to report ongoing spatial disorientation due to LEDs, although others report spatial disorientation of shorter duration, and this individual was the only person not to report dry eye and was one of two people not to report pain/soreness of eye muscles as a typical symptom. It is interesting to speculate that migraine symptoms might influence the scope of LED sensitivity symptoms, such as the experience of nausea during a migraine possibly predisposing one to ongoing nausea due to LED sensitivity, however, nothing can be concluded from one individual.

As more people complete the survey, it will be interesting to see whether there is any actual correlation between migraine or other health conditions and the incidence, scope or severity of LED sensitivity symptoms.

A possible role for eye convergence insufficiency or eye misalignment for some people with LED screen sensitivity

Eye convergence is the coordinated inwards turning of both eyes when focusing on something close (see Background: Normal eye responses to light). Some people are born with convergence insufficiency. Other people might later acquire convergence insufficiency - for example, convergence insufficiency is a common symptom of concussion. Symptoms of convergence insufficiency are eyestrain, headaches, trouble reading, trouble concentrating, double vision, and a tendency to close one eye. Cognition is not affected by convergence insufficiency - there is a vision problem, but not an underlying learning disability. Routine eye exams usually do not test for convergence insufficiency. Prismatic lenses, patching an eye, or the use of eye exercises are methods for correcting or compensating for convergence insufficiency.

Strabismus is a form of eye misalignment, where the turnings of the two eyes don't match. One eye may be turned too much inwards, outwards, up, or down. People may have strabismus beginning in early childhood or may acquire it later, for example, following a stroke or head injury (Martinez-Thompson et al., 2014). It is often caused by an abnormality in neural signaling rather than by a problem with the function of the actual eye muscles. Symptoms of strabismus are eyestrain, headaches, blurry vision, double vision, poor three-dimensional vision, and fatigue. Cognition is not affected. Prismatic lenses, patching an eye, eye muscle surgery, or the use of eye exercises are methods for correcting or compensating for strabismus.

Additionally, some people have a form of "hidden" strabismus called heterophoria that's particularly difficult to detect because their eyes often look fine, but one eye will start to drift while they're focusing on something. The natural resting positions of the eyes with the lids closed are not aligned. People with heterophoria have difficulty coordinating parts of the accommodation-convergence reflex: (1) changing the shape of the lens to focus and (2) eye convergence while focusing on near objects.

One out of the 11 survey respondents reported that using prismatic lenses helped "a lot" to prevent their symptoms when using LED screens (Fig 107). This is the same individual who was the only one to report that using blue-filtering yellow or orange computer glasses helped "a lot" to prevent their symptoms when using LED screens (Figs 78, 106). This individual did not try any of these lenses with ambient LED light. They report additional symptoms not typical of eye convergence or strabismus symptoms (sleep abnormalities, muscle seizures, pressure that feels like swelling, and swelling of the eyelid or near the eyelid).

Two other survey respondents indicated that they tried prismatic lenses and that they didn't help to prevent their symptoms from either LED lights or LED screens (Fig 107). One of these individuals in a free response described trying therapy that corrected a convergence issue (of unreported severity), but this therapy didn't help to prevent their LED symptoms.

Although convergence insufficiency or strabismus doesn't necessarily seem to be a problem for all people with sensitivity to LED lights or screens in the LED Sensitivity Survey, it seems quite possible that eye convergence insufficiency or strabismus could be factors in the sensitivity to LED screens in a subset of individuals reporting sensitivity. Perhaps these conditions might somehow exacerbate the nervous system's sensing of flicker. While there is only one response out of 11 so far in the LED Sensitivity Survey that seems potentially consistent with an issue of eye convergence or misalignment, there is wider discussion of the topic and more anecdotal reports on the LEDstrain.org forum. It would be helpful to acquire much more data to begin to assess the frequency and scope of various eye movement/alignment insufficiencies in contributing to LED symptoms.

There are many reasons why reading on a screen is more taxing than reading on paper and problems associated with eye convergence insufficiency or strabismus could be amplified when reading on a screen, especially given the multiple sources of screen flicker (visual stress when reading is reviewed in reference to fluorescent light and CRT screens in Wilkins, 1995 and an updated review referencing LED lighting is provided in Wilkins, 2021).

Binocular vision dysfunction (BVD) and prismatic lenses

"Binocular vision disorder" is a collective description that includes the variety of vergence and accommodation conditions in which there isn't complete coordination of vision from the two eyes, including convergence insufficiency and strabismus. "Binocular vision dysfunction" (BVD) could be used in the same sense as "binocular vision disorder" but is often associated with the work of optometrists using prismatic lenses to alleviate vision, eyestrain, dizziness, and headache symptoms, particularly in concussion patients, that the optometrists attribute to vertical heterophoria, the misalignment of the eyes in the vertical plane. As of December 18, 2021, there are 13 entries for "binocular vision dysfunction*" in PubMed, with all in 2007 or later, many of which are associated with work related to vertical heterophoria. There is not an objective way to diagnose vertical heterophoria through an eye exam. One group that has been at the forefront of BVD research, makes a diagnosis of vertical heterophoria if a patient reports that their symptoms improve following the use of prismatic lenses (Doble et al., 2010). This group reports that 43 post-concussion patients who they diagnosed with vertical heterophoria because they reported symptom improvement following the use of prismatic lenses, showed evidence of symptom improvement on a survey following the use of prismatic lenses. This circular reasoning is a serious limitation in interpreting the work (Barton & Ranalli, 2021), along with other significant limitations of the study design, such as the lack of a control group that did not receive the lenses, the lack of inclusion of patients who did not show symptom improvement with prismatic lenses, and the inclusion of only one optometrist's work (Doble et al., 2010). They have also performed a similar study of an additional 38 post-concussion patients that has similar limitations (Rosner et al., 2016). However, these serious study limitations do not rule out the possibility that some patients are helped by prismatic lenses.

It also isn't clear how well the symptoms attributed to vertical heterophoria might align with the symptoms of those who experience LED symptoms. The Vertical Heterophoria Symptom Questionnaire (Doble et al., 2010) and the nearly identical Binocular Vision Dysfunction Questionnaire (Rosner et al., 2016) only address light sensitivity by querying sensitivity to "glare or bright lights" together in one question and do not ask specifically about computer use, but only have questions that include computer use as one example in a list of other "close-up" activities that also includes reading and writing. It would be interesting to know whether there was any overlap between symptoms associated with light or screen flicker and the symptoms that could be improved by the use of prismatic lenses. Overall, these questionnaires have some overlap with reports of "LED" symptoms, particularly in terms of headache, eyestrain, anxiety, and the possibility that "dizziness" might be similar to spatial disorientation, but do not overlap in a number of other areas. While anecdotal reports that prismatic lenses help some people with post-concussion symptoms or with LED symptoms provide some hope, much more rigorous scientific analysis of the utility of this intervention is needed. Even if it were shown to be an effective treatment strategy for some patients with LED symptoms in future research, the lack of a screening method for whether prismatic lenses might help someone, other than their trying multiple prismatic lenses, is a serious barrier to this becoming a widespread intervention to help prevent symptoms from LED lights and screens.

LED sensitivity: Cautions concerning mental health

Among LED Sensitivity Survey respondents, 8/11 report anxiety greater than 3 on a scale of 0-10, 6/11 report irritability, and 4/11 report depression being caused by exposure to LED lights and/or screens (Figs 41-42). It is hoped that individuals struggling with mental health issues will be able to find the appropriate professional help for dealing with these conditions. Anxiety and depression are also sometimes symptoms of dry eye disease, migraine and concussion. Irritability is also sometimes a symptom of migraine and concussion. Neural signaling from trigeminal neurons through the hypothalamus in these photophobia-related conditions is thought to be a physiological cause of anxiety, irritability, and depression (reviewed in May and Burstein, 2019). The substantial overlap of other symptoms between LED sensitivity and dry eye, migraine, and concussion support the hypothesis that similar neural signaling might be a physiological cause of anxiety, irritability, or depression for those with LED sensitivity as well.

It is also hoped that the occurrence of these mental health symptoms in some, but not all, survey respondents will not be cited as an excuse for dismissing their reports of health problems caused by LED lights and/or screens as not "real." If anything, the occurrence of these mental health symptoms is a further indication that the physiological basis of LED symptoms could overlap with that of dry eye, migraine, and concussion since these mental health symptoms are also shared by all of these conditions.

Note: Spatial disorientation may somewhat mimic a feeling of anxiety

Spatial disorientation can occur when viewing LED light or screens, possibly due to being somewhat disoriented when visually sensing light flicker. The feeling of spatial disorientation might only be just barely noticeable or could be strong enough that it is quite noticeable. Spatial disorientation can also be an ongoing LED symptom even after the LED light/screen is off. When assessing feelings of anxiety, it may be tricky because the sensation of spatial disorientation is an off-putting sensation of being disconnected from the environment, that, in my experience, feels somewhat similar to, but not exactly the same as, the jittery feeling that can accompany an acute feeling of anxiety. Physicians should be careful in how they assess patients to be able to distinguish between these possibilities. For example, a vague feeling of uneasiness in the presence of LED lights could be a minor sense of spatial disorientation because the flicker is visible at a subliminal level, rather than an anxiety response or psychosomatic response to the light. Physicians should not jump to the conclusion that people feel uneasy in LED light because they're worried about getting triggered by the light and are therefore feeling anxious. They could instead feel uneasy in the LED light because the light flicker is slightly disorienting for them, similar to, but less extreme than, how more obvious strobe lights are quite disorienting. (Also see Self-tests).

Overcoming a history of bias, skepticism and dismissal of reports of neurological symptoms

People experiencing LED symptoms often encounter skepticism from lighting professionals and from physicians that their symptoms have a physiological basis. Contributors to LEDstrain.org report that they don't know of any objective test for their conditions. The experiences of people with LED symptoms are reminiscent of post-concussion syndrome (PCS), both in terms of many of the types of symptoms (see above), the lack of a biomarker test, and the skepticism with which medical professionals sometimes consider patients' experiences.

Historically, there has been bias in the medical community against believing patients describing their ongoing PCS symptoms. PCS has been suggested to sometimes be a "somatic symptom disorder," in which the condition results from the patient's excessive focus on or worry about their physical symptoms. There are also suggestions in the medical literature that subsets of patients exaggerate or manufacture PCS symptoms because they are wanting to avoid work or school, are attention-seeking, or are engaged in legal action. For example, an abnormal result in the Romberg test - whether a patient can maintain balance with their eyes closed - has been shown to be a clinical predictor of a long duration of post-concussion syndrome. However, even the researchers conducting this study suggest, without first-hand evidence, that the extra upper body movement that might be observed in an abnormal Romberg test in children and adolescent post-concussion patients to be a sign of exaggerating or feigning symptoms and therefore perhaps an indication of an underlying psychiatric condition, rather than evidence of physiological issues stemming from the concussion in a subset of patients (Howell et al., 2019). They cite an earlier study from their own research group that links abnormal Romberg tests in children with post-concussion syndrome to a higher probability of poor performance on a computerized word memory test, which they interpret as evidence of feigning symptoms (Provance et al., 2014).

Skepticism among medical professionals about whether there is a physiological basis for post-concussion syndrome, may have arisen partly because there are usually no objective clinical tests or biomarkers for PCS (Tator, 2013). For example, neuroinflammation can't usually be detected clinically. However, in recent years, a significant amount of evidence has supported there being a physiological basis for PCS. For example, abnormal changes in the brain that are consistent with dementia and thought to have resulted from the neuroinflammation and trauma of concussions have been apparent upon autopsy in cases where there were no tests that could confirm PCS during the patient's lifetime (Mayer et al., 2017). Given the skepticism that patients with PCS symptoms arising from a physical brain injury have encountered, it is not at all surprising that patients reporting LED symptoms face similar or more skepticism, particularly since the light triggering the symptoms in some people doesn't seem to bother everyone and isn't linked to an obvious physical injury.

The absence of objective biomarkers of nervous system or neuroinflammatory changes (other than upon autopsy) in post-concussion syndrome or for the photophobia experienced in migraine or dry eye disease raises the likely possibility that there also may not be readily measurable, objective biomarkers to diagnose sensitivity to LED flicker or to detect potential nervous system sensitization or inflammatory signaling.

Further complicating matters for patients with LED symptoms is another long-standing bias in the medical community to believe that photophobia, sensitivity to light, has a purely psychological, rather than physiological basis. Even through research in recent years has demonstrated the physiological basis of photophobia syndromes, there may still be difficulties for patients. Medical professionals might still think that the "sunglasses sign," wearing tinted glasses in the clinic, is a sign of a purely psychiatric disorder or a sign that the patient has made up the symptoms to seek attention, rather than evidence that the light in the clinic is having a genuine impact on the patient's physical health. Howard & Valori (1989), describe clinician's use of the sunglasses sign, as an indicator that a patient does not have a physiological condition, as a long-standing, widespread assumption in clinical practice that has not been validated by evidence. The preponderance of currently-available evidence suggests that there are physiological causes of photophobia and that the vast majority of patients exhibiting the sunglasses sign have photophobia with a genuine physiological basis (reviewed in Digre and Brennan, 2012 and Katz and Digre, 2016). Additionally, while some conditions with photophobia may also have anxiety or depression as symptoms in subsets of patients, Katz and Digre (2016) say from their perspective as ophthalmologists that they have never encountered any patients that present in the clinic with photophobia because they have a psychiatric condition.

What should we call this kind of light sensitivity?

We need a way to describe this sensitivity to others that conveys both the severity of the issue and accurately identifies its cause.

It's misleading to call this "LED sensitivity," since it doesn't seem to be sensitivity to all LEDs and for some people like me, completely flicker-free LEDs are actually the only kind of artificial light that doesn't trigger my sensitivity at all. If we had more evidence to support the hypothesis that flicker is the triggering factor, we could call this "flicker sensitivity" but much more evidence is needed to support this idea. Also, "sensitivity" doesn't convey how severe the symptoms can be, how much it can of an impact this condition can have, or the possibility that changes to the brain might be taking place in the form of sensitization and neuroinflammation. The triggering LED lights or LED screens seem to be causing longer-term injury to people, rather than being simply a transient source of pain or irritation.

It isn't correct to describe these symptoms as "migraine" or "eyestrain" because the wide range of symptoms doesn't fit either of those conditions very well.

Each person separately describing their own symptoms can be helpful for them to make other people understand about their particular situation, but it would be more helpful to advocate for ourselves as a group to have a description that is more universal.

This will require more thought from the community experiencing these symptoms.


  • This survey was first distributed in June 2021 on the LEDStrain.org forum. The demographics of forum users, which seems to be partially biased toward individuals with experience in computer technology, may not reflect the demographics of the general population.

  • As of the time of writing, there are only 11 survey responses from individuals who report LED-caused symptoms, so it is not reasonable to draw any firm conclusions from the limited data set.

  • Because the survey is publicly available and anonymous, individuals are able to take it more than once, individuals may provide false or incomplete information, and there is likely to be bias, of an unknown nature, among the individuals who complete the survey.

  • The time necessary to complete the survey is a barrier, that is both useful in deterring spam responses, and detrimental in deterring completion by genuine respondents. Sometimes, it seems as though some respondents click the same response for every item in a list without reading them or otherwise randomly choose answers because there are inconsistencies in the logic of answers for some people.

  • This survey being online is both useful for widespread distribution, but is also a barrier for individuals who have difficulty using screens.

  • While the survey's author tried to create questions that could capture the diversity of experiences already described on the LEDStrain.org forum, the author also is biased by her own experiences with LED lights and screens.

  • This survey is not particularly scientific in that it is not well-controlled. There is not a way to distribute the survey to a random sample of the population and to ensure that each person completes it only once.

  • The results of this survey are not being published in a peer-reviewed journal. The author's hypothesizing about what might be happening is done with greater speculation than might be done in the scientific literature, especially given the relatively few responses so far to this survey. The decision has been made to do so in order to help to identify potential future areas of research.


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