The Health Effects of Flickering Light
People vary widely in how they are affected by flicker, the repetitive variation of light intensity. When flicker is slow enough to be obviously visible to anyone, most people find it to be mildly annoying, but some people's nervous systems initiate harmful physiological responses that may include pain, headache, eyestrain, disorientation, nausea, and, rarely, even seizures. When flicker is fast enough that most people aren't consciously aware that their eyes still see it, seizures are unlikely, but some people may still experience pain, headache, eyestrain, disorientation, nausea, inhibition of short-term memory, fatigue or insomnia. People's nervous systems vary considerably in their response to flicker. Most people's nervous systems will respond to repetitive stimuli, including flicker, by decreasing the neural signaling that the stimulus triggers, a phenomenon called "habituation" or "adaptation" that is thought to limit unnecessary overstimulation of the nervous system and limit unnecessary distraction. A common example of this is how we generally don't continually think about the sensation of wearing clothing because the nervous system quickly habituates to the sensation. Most people's nervous systems similarly become habituated to flicker so that the nervous system responds less to flicker with repeated exposure. However, some people's nervous systems not only don't become habituated to flicker, they actually become even more sensitive to flicker with more flicker exposure. These are the people whose health is most at risk from the flicker of ambient light or screens.
Humans have evolved in a world, that until quite recently, did not include regularly repetitive flicker. Incandescent lights first introduced a fairy mild form of flicker that people weren't consciously aware of seeing and that didn't seem to bother people, but the development of fluorescent lights first introduced a harsher form of flicker that was linked to headaches, eye strain, and other adverse health effects. Special electronic ballasts for fluorescent lights were then introduced to reduce the flicker in the late 1980s and the health impacts were also reduced. Even though light flicker has long been known to cause adverse health effects, in the last decade flicker has become increasingly ubiquitous in the human environment from ambient LED lights and LED screens. There has also recently been increased use of flicker in computer software techniques for improving apparent screen image quality, such as the use of rapid color flicker to render larger color palettes, smooth color gradients, and create sharper-looking images.
The near-ubiquity of flicker in ambient LED lights available now in 2021 in the U.S. is particularly unfortunate because LED lights do not have to flicker - it's a choice made by lighting manufacturers. In fact, the advent of LED lights was greeted with hope by those who had previously studied the adverse health effects of fluorescent light flicker because of the fact that, unlike fluorescent lights, LED lights could be manufactured to not have any flicker at all. This just required the inclusion of an extra circuit in the LED bulb that would eliminate the flicker by making the current supplied to the LED light constant. Some manufacturers, including Philips, initially chose to produce completely flicker-free LED bulbs during the first significant wave of consumer LED bulb mass-production around 2013. Very large LED fixtures like new street lights often still incorporate completely flicker-free circuitry today.
One serious problem with LED lights is that no one knew during their initial development and no one knows now what, if any, level of light flicker is safe for the sensitive segment of the population. No one even knows what percentage of the population may be sensitive to flicker. A second serious problem is that even though LED flicker was hypothesized to be a potential health risk, no one provided any mechanism to assess the health impact of LED lights, LED screens, or the software-based introduction of flicker. LED bulbs and screens that produce flickering light have not undergone the scrutiny of clinical trials for medical devices, even though it was well-known at the time of their development that they can have medical effects. In the absence of any significant scientific data, and in the absence of significant regulation, major lighting manufacturers, including Philips, have shifted to currently only produce consumer bulbs that flicker. In most places, there is also no requirement that lighting manufacturers report flicker statistics for their bulbs, so consumers cannot obtain the information needed to make informed decisions. The lighting industry decided in 2016 that they wouldn't even use the word "flicker" anymore unless they were referring to obviously visible flicker, and instead use "temporal light modulation" to refer to the rapid flicker produced by typical LED lights (CIE TN-006). This can create significant confusion in communicating about this issue with the general public since the lighting industry can now claim that flickering LEDs do not flicker, according to their own limited definition. Lights with quite significant temporal light modulation are currently marketed as "flicker-free."
Since there are no medical studies of the health impacts of flicker from LED lights or screens, and since the ubiquity of environmental LED flicker is a relatively recent phenomenon, medical professionals currently don't have any resources in the medical literature to inform their recognition, testing, or treatment of affected patients. There is no centralized mechanism for reporting cases of patients who may have adverse physiological reactions to light flicker. There is an added barrier to recognition of the problem in the medical community, as historically many physicians thought patients displaying signs of photophobia have psychological, rather than physiological, conditions. Although recent neurobiology research now supports physiological, rather than psychological, causes of photophobia, patients still may face historical biases that cause their physicians to automatically dismiss their experiences as "all in their heads," especially since they are presenting at the clinic with a novel condition not yet in the literature.
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/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 or depression, and various additional symptoms that suggest changes in the nervous system due to sensitization. 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 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 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.
I strongly advocate for completely flicker-free LED lights because I believe they are both healthier than other artificial lights, and the energy-savings of LEDs is good for the environment. I also urge the lighting and computer industries, engineers, scientists, medical professionals, politicians, and individuals to take action to begin to protect people sensitive to light flicker from being exposed to flicker in public places, in residences, in education, in workplaces, or on screens. Analyses have shown that there is no difference in the retail cost of LED lights that are completely flicker-free compared to those that flicker (Wilkins, 2021), however by continuing to allow the manufacture and installation of flickering LED lights, there is a human cost in terms of the health and cognitive function of a segment of the population and a growing future monetary cost of replacing flickering LED lights with completely flicker-free LED lights once the health effects of LED flicker are widely recognized. I advocate for making this shift from flickering LEDs to completely flicker-free LEDs now.
If you are fortunate enough to be in the majority of people who are relatively insensitive to light flicker, I ask you to please consider the experiences of people sensitive to light flicker with an open mind. I was personally surprised to learn that the pain or disorientation that I can experience from light flicker or repetitive patterns, often immediately and without warning, isn't everyone's universal experience, as it feels like such a fundamental physiological response to me, like the pain felt in the instant of touching a hot stove is a nearly-universal physiological response. Please be open to the possibility that different people experience light flicker quite differently.
Jennifer Hackett, PhD
My degree in Human Genetics and Molecular Biology from Johns Hopkins University School of Medicine included relatively little training in neuroscience, so I do not claim any particular expertise in that field. However, I am a scientist who has been fortunate enough to train at some of the most respected science research institutions with some of the best scientists in the world (learn more about me here). I have attempted to apply my skills as a scientist to this work. I discovered in 2018 that I am very sensitive to the flicker of LED lights and screens and have learned that too many other affected people are in urgent need of assistance because they cannot complete their education or keep working in their chosen careers due to the adverse health effects of some LED lights and screens. It isn't good practice to include oneself as a subject of research and it is preferable to conduct surveys that are sponsored by a research institution and that have been created by experts. However, I waited for three years hoping someone else would start to do this research properly and finally decided that the only way to get it started any time soon was for those of us who are affected to start it ourselves. The need was too great to wait any longer.
(The opinions expressed on this site are my own and do not reflect the views of my employer).