"Light works as if it’s a drug, except it’s not a drug at all."

-George C. Brainard, PhD, director of the Light Research Program at Thomas Jefferson University in The New York Times (2011).

LED Lighting, Screens and Health

Flicker Below 100 Hz:

Scientific Literature

Is flickering light unhealthy?

Visible flicker <100 Hz: Biological effects of light flicker in the scientific literature

Visible flicker <100 Hz is slower than is usually found in modern lighting sources, unless they are malfunctioning. However, most of the research on the health effects of flicker studies this slower flicker. It is unclear to what extent observations of the biological effects of flicker <100 Hz might also be relevant to the ≥100 Hz flicker found in modern artificial lighting. However, there are multiple potential sources of <100 Hz visible flicker from LED screens (see Background: LED Screens) so the research below could be directly relevant to understanding some of the health effects of LED screens. Additionally, many LED string (holiday) lights have started to be designed to have 60 Hz flicker, making the research below definitely relevant to those products (see the NPR ShortWave podcast linked below the Public Health Risks)

Scott, A., McCoy, B. Ramirez, R. It's not just you: Christmas lights look different now, and can give you headaches. December 22, 2023. National Public Radio: Short Wave.https://www.npr.org/2023/12/22/1198908957/led-lights-flicker-headache

Photosensitive epilepsy: Visible 3-70 Hz flicker causes seizures

Visible light flicker causing seizures in individuals with photosensitive epilepsy (reviewed in Wilkins, 1995) is a widely-recognized phenomenon and is the reason for warnings of strobe light use at live events and for the avoidance of certain kinds of visible light flicker in animated videos. Table 1 of the IEEE report (IEEE std 1789, 2015) summarizes data on seizures and EEG abnormalities being caused by visible flicker in people with photosensitive epilepsy. The IEEE report also describes how people with photosensitive epilepsy may have seizures due to either visible flicker in the 3 Hz to 70 Hz range or repetitive geometric patterns. This includes flickering artificial light, flicker from old cathode ray tube (CRT) televisions or computer screens (reviewed in Wilkins, 1995), and flicker of sunlight through trees when driving or flicker of light reflected from water. The report further summarizes factors influencing whether visible flicker may trigger a seizure.

Seizures and other adverse biological effects of flicker are more likely to occur when:

the flicker is brighter
the contrast in the dark and light phases of the flicker is greater; lower background lighting creates more contrast.
the percentage of the retina that is exposed to flicker is greater - full-field lighting is worse than a point light source.
flicker in the center of the visible field is more likely to cause seizures than flicker in the periphery, even though people tend to notice peripheral flicker more.
certain wavelengths of light flicker, especially deep red.
multiple lights are arranged in repetitive linear sequences or 2-dimensional arrays. Arrays of lights may cause seizures even if they don't flicker.

If exposed to visible flicker, patients are advised to cover one eye with a hand, turn away from the flicker, and leave the area with flicker. Simply closing an eye is insufficient because the flicker can be observed through the eyelid.

Seizures in response to flicker have also been reported for flicker in television animations. These include reports of 685 children in Japan experiencing seizures due to the broadcast of a cartoon with 12.5 Hz flickering of red and blue frames in 1997 (Harding, 1998) and reports of seizures in response to a flickering advertisement for the 2012 Olympic Games in the UK in 2007.

Glendinning, L. Please look away...it's the 2012 logo. The Guardian, June 6, 2007. https://www.theguardian.com/media/2007/jun/06/marketingandpr.olympics2012

Harding G.F. TV can be bad for your health. Nat Med. 1998 Mar;4(3):265-7. https://www.nature.com/articles/nm0398-265.pdf?origin=ppub

The Institute of Electrical and Electronics Engineers, Inc. IEEE Std 1789™-2015: IEEE Recommended Practices for Modulating Current in High-Brightness LEDs for Mitigating Health Risks to Viewers. 2015. http://www.bio-licht.org/02_resources/info_ieee_2015_standards-1789.pdf

Wilkins, A.J. (1995) Visual Stress. Oxford University Press. 194 pp. http://www1.essex.ac.uk/psychology/overlays/book1.pdf Warning: This book contains multiple patterns that may bother sensitive individuals. The book warns those with epilepsy and migraine not to look at the frontispiece. Patterns in the book triggered my "LED" symptoms when I ignored the warning.

The 50-60 Hz visible flicker of magnetically-ballasted fluorescent lights is more often observed by office workers reporting headache or eyestrain; headache and eyestrain are reported at very high rates

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. In addition, 24% reported that they could see the lamps flickering and 10% reported that they could see the flicker on their work. Those reporting headache were more likely to report seeing flicker of lamps (p = 0.0016) or on their work (p = 0.0019). Those reporting eyestrain were more likely to report seeing flicker of lamps (p = 0.0000014) or on their work (p = 0.047). [Chi-square calculations with the Yates correction for continuity from the original paper were recalculated and p values were determined in Excel- see my analysis and graphs of this portion of the Brundrett (1974) data here]. This study also found that younger people were more likely to see the flicker and to report headaches than older people (39% report seeing lamp flicker and 60% report headache at work among the group less than 20 years old (n=92) and this trends downward until 13% report seeing lamp flicker and 18% report headache in the group in their 50s (n=85); none of the group in their 60s (n=16) report either seeing flicker or getting workplace headache. However, no information about the gender composition of study participants was provided, so it is unclear how these data might correlate with other studies generally showing higher rates of headache in women than in men, but with lower rates of headache in individuals under 30 than over 30 years old. Whether exposure to window sunlight was related to seniority was also not addressed. The rate of eyestrain varied only slightly with age in this study, although the trend was also for eyestrain to be higher in younger workers.

Brundrett, G. W. Human sensitivity to flicker. Lighting Research & Technology. 6, 127-143 (1974). https://journals.sagepub.com/doi/abs/10.1177/096032717400600302?journalCode=lrtb

People reporting headaches only when working under magnetically-ballasted fluorescent lights show more sensitivity to 5-90 Hz visible flicker on EEGs than controls

Brundrett (1974) reviews prior research showing that electroencephalogram (EEG) waveforms spike in time with the flashes of up to at least 100 Hz flickering light, and Brundrett conducts such a study on 10 people and shows that their EEG responses to flicker segregate them into 2 groups. One group contained only 4 people who reported experiencing headaches only while at work (but did not have headaches at other times) and the other group contained 5 people without headaches and 1 person who reported having headaches at work. In the study, flicker of a lamp was increased from 5 Hz to 90 Hz in 5 Hz steps. The person was exposed to the flicker for 20-30 seconds for each frequency below 30 Hz, with the amount of time of exposure increasing to 120 seconds by 90 Hz. The frequencies were tested in order, beginning at 5 Hz, with the entire testing session taking 2-3 hours. The EEG response to the flicker started at about the same level for both groups in the first test at 5 Hz and then the EEG response gradually declined in the headache group, but more sharply declined in the non-headache group. By the end of the experiment at 90 Hz, the EEG response of a headache group member was about 10 times higher than that of a non-headache group member. It isn't clear how much of the decline in EEG signal over time was due to an attenuation/habituation response of the brain to the repetitive signal, either generally over the entire time course or during the increasingly long individual tests, and how much of the decline in EEG signal was due to the increase in flicker frequency. However, it is clear that there were differences in the EEG response to flicker between those reporting headache only at work and those not reporting headache, with greater brain sensitivity to flicker in the headache group. Brundrett even notes that the non-headache group tended to be prone to falling asleep toward the end of the testing session because there wasn't anything to keep their attention, whereas the headache group tended to report increasing discomfort over the course of the test and were not prone to falling asleep. It is very interesting that an EEG difference is found in this study between two "normal" groups of people, since none of the individuals report getting headaches outside of work. The gender composition of the test subjects was not provided.

Brundrett, G. W. Human sensitivity to flicker. Lighting Research & Technology. 6, 127-143 (1974). https://journals.sagepub.com/doi/abs/10.1177/096032717400600302?journalCode=lrtb

Visible flicker causes migraines

The IEEE report (IEEE std 1789, 2015) also reviews self-reports by patients of visible flicker causing migraines, including malfunctioning fluorescent light flicker, flicker in TV and movies, and various other forms of visible flicker

People who get migraines are more sensitive to visible flicker than normal individuals and become increasingly sensitive with more flicker exposure

In a study of the ability to observe flicker contrast in migraine patients (Karanovic et al., 2011), full-screen 10 Hz visible flicker on a CRT computer monitor was more aversive to migraine patients than to normal controls. Discomfort scores correlated with the number of years of having had migraines.

In a different set of tests involving fixation on a 10 Hz flickering spot (Karanovic et al., 2011), while normal controls become less sensitive to flicker with repeated exposure (habituation/adaptation), migraine patients become more sensitive to high-contrast flicker with more exposure to flicker. Only 14 migraine patients and 14 normal controls completed the study which required about 5 hours split over 5-6 sessions, but nine additional migraine patients and three additional controls dropped out early. Three of the migraine patients cited increased migraines and two other migraine patients started taking antidepressant or pain medication. Thabet et al., (2013) extended this study and also observed greater flicker sensitivity in migraine patients and sensitization with more exposure to high contrast flicker. The extra flicker exposure in these studies consisted of a 2 or 3 minute adaptation period of additional flicker exposure. It would be interesting to know how long-term flicker exposure affects sensitivity in individuals prone to flicker sensitivity.

McColl (2002) shows higher visual discomfort when viewing grating patterns in migraine patients compared to normal controls, especially if the grating patterns flickered. The introduction to this chapter also reviews the literature on visual discomfort experienced by migraine patients when viewing repetitive patterns. Wilkins (1995) reviews literature on how spatially repetitive patterns can cause visual discomfort, seizures, and headaches.

Angelini et al. (2004) showed that there is increased EEG alpha band phase synchronization of brain waves in migraine patients exposed to 9 Hz, 24 Hz, and 27 Hz flicker, but there is decreased brain wave synchronization in normal controls.

de Tommaso et al., 2014 review evidence of a lack of habituation to repetitive stimuli for migraine patients and research on how brainwaves become synchronized during a migraine.

Together, the above reports suggest that migraine patients are more sensitive to visible flicker than normal controls, lack a normal habituation response to high contrast visible flicker, and have brain wave synchronization in response to visible flicker, while normal controls have decreased synchronization. None of these data are based on studies of the effects of the faster flicker produced by LED lights, but they raise the possibility that some of these trends may potentially be relevant with faster flicker.

Angelini L. et al. Steady-state visual evoked potentials and phase synchronization in migraine patients. Phys Rev Lett. 2004;93(3):038103. https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.93.038103

de Tommaso M. et al. Altered processing of sensory stimuli in patients with migraine. Nat Rev Neurol. 2014;10(3):144-155. doi:10.1038/nrneurol.2014.14

https://orbi.uliege.be/bitstream/2268/182135/1/de%20Tommaso%20et%20al%20-%20Nat%20rev%20Neurol%202014.pdf

Karanovic et al. Detection and discrimination of flicker contrast in migraine. Cephalalgia 31 (2011) 723-36. https://journals.sagepub.com/doi/pdf/10.1177/0333102411398401

McColl SL. Interictal visual system function in migraine: a psychophysical approach. PhD thesis. Montreal: McGill University, 2002.https://escholarship.mcgill.ca/concern/theses/pg15bf456?locale=en

Thabet et al. The locus of flicker adaptation in the migraine visual system: A dichoptic study. Cephalalgia 33 (2013) 5-19. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4011828/pdf/nihms4392.pdf

Flicker vertigo: Reports from the aviation industry

"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. 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 described in the U.S. Air Force Flight Surgeon's Guide and case reports, human and monetary cost in aviation, causes, and military research are reviewed by Clarence E. Rash, a physicist at the U.S. Army Aeromedical Research Laboratory. Flicker frequencies between 4 Hz and 20 Hz have been most associated with flicker vertigo.

Rash, Clarence E. Awareness of causes and symptoms of flicker vertigo can limit ill effects. Flight Safety Foundation: Human Factors & Aviation Medicine. 51(2) March-April 2004. https://flightsafety.org/hf/hf_mar-apr04.pdf

Visible 40 Hz flicker causes neuroinflammation: flicker as a noninvasive therapy to alter brain chemistry

Visible 40 Hz light flicker has been suggested as a possible therapy for Alzheimer's disease because the flickering light causes neuroinflammation that causes clearance of the plaque-forming amyloid beta peptide in the brains of mice (Iaccarino, 2016 and reviewed in Thomson, 2018). Garza et al., 2020 further characterize the distinct neuroimmune signaling profile created by 40 Hz flicker in the brains of mice. While the ability to use flickering light to cause brain inflammation is interesting as a potential Alzheimer's disease therapy, it raises the question of whether the flicker of ambient LED lights or screens might cause neuroinflammation in some people (no studies have been done). If this is the case, it would be important to study the possible short and long term health effects of causing inflammation in the brain. The above studies did not test flicker faster than 40 Hz.

Garza KM, Zhang L, Borron B, Wood LB, Singer AC. Gamma Visual Stimulation Induces a Neuroimmune Signaling Profile Distinct from Acute Neuroinflammation. J Neurosci. 2020 Feb 5;40(6):1211-1225. https://doi.org/10.1523/jneurosci.1511-19.2019

Iaccarino HF, Singer AC, Martorell AJ, Rudenko A, Gao F, Gillingham TZ, Mathys H, Seo J, Kritskiy O, Abdurrob F, Adaikkan C, Canter RG, Rueda R, Brown EN, Boyden ES, Tsai LH. Gamma frequency entrainment attenuates amyloid load and modifies microglia. Nature. 2016 Dec 7;540(7632):230-235. https://doi.org/10.1038/nature20587

Thomson H. Wave therapy: How flashing lights and pink noise might banish Alzheimer's, improve memory and more. Nature. 2018 Mar;555(7694):20-22. https://doi.org/10.1038/d41586-018-02391-6

Visibly flickering 60 Hz light appears to be brighter than continuous light

The Broca-Sulzer effect (1902; see Corwin & Green, 1978) describes how humans consciously perceive a flashing light to be brighter than a comparable continuous light. This is the rationale for using flashing lights in safety beacons. Jinno et al. (2008) showed that 60 Hz flickering light appeared brighter than non-flickering light to two normal male subjects. The conclusion of Jinno et al. that introducing flicker into LED lights could be adopted as an energy-saving strategy since people sense the flicker as extra-bright, highlights the idea that current LED light flicker may in some circumstances have been desired by lighting manufacturers because it can be sensed by the brain.

Corwin, T.R. & Green, M.A. The Broca-Sulzer Effect in a Ganzfeld. Vision Research, 18, 1675-1678 (1978). https://www.visualexpert.com/Publications/Broca-Sulzer%20effect%20in%20a%20Ganzfeld.pdf

Jinno et al. Effective illuminance improvement of a light source by using pulse modulation and its psychophysical effect on the human eye. Journal of Light and Visual Environment. 32 (2008) 161-169. https://www.jstage.jst.go.jp/article/jlve/32/2/32_2_161/_article

Next section: Flicker ≥100 Hz in the scientific literature

Health Effects of LED Lights and Screens (Introduction)