Key takeaways
Bright light during the day and dim light in the evening is the best-supported pattern for a stable body clock and good sleep
Expert recommendations call for at least 250 lux melanopic EDI at the eye during the day, under 10 lux in the 3 hours before bed and under 1 lux while sleeping
Blue-light glasses have uncertain effects on sleep, while dimming lights, moving screens away from bedtime and getting outdoors in the morning are cheap and sensible
Light is the strongest signal your body clock receives. It tells your brain what time it is, which shapes when you feel alert, when melatonin rises, when you get sleepy and, indirectly, how well you recover from training. Modern life tends to get it backwards: dim offices by day, bright screens and kitchens late at night.
How light sets the clock
Your master clock sits in the suprachiasmatic nuclei of the hypothalamus. A 2019 review by Blume, Garbazza and Spitschan in Somnologie explains that this clock is aligned to the 24-hour day by signals from the retina, particularly from light-sensitive cells containing melanopsin, which respond most to blue-green light.
Key points from the review:
- Light in the morning generally shifts the clock earlier; light in the evening and night shifts it later.
- Light at night suppresses melatonin and has been shown to alter circadian rhythms and sleep.
- Artificial light has changed our evening environment, which may raise the risk of circadian rhythm sleep-wake disorders, though the exact link "remains to be established".
- Light therapy is an effective, non-invasive treatment for some sleep and mood problems, with few side effects.
How much light, and when?
In 2022 an international group of experts led by Brown published consensus recommendations in PLoS Biology. They used melanopic equivalent daylight illuminance (melanopic EDI), a measure of how strongly light acts on melanopsin.
| Time | Recommendation (at eye level) | Practical translation |
|---|---|---|
| Daytime | At least 250 lux melanopic EDI | Go outdoors, sit near windows, use bright indoor lighting |
| Evening (from 3 h before bed) | No more than 10 lux melanopic EDI | Dim, warm lamps rather than bright overhead lights |
| Night (sleeping) | No more than 1 lux | Dark bedroom, cover standby LEDs, blackout blinds |
Typical indoor lighting often falls short of the daytime target, while outdoor daylight, even on cloudy days, is usually much brighter. That's why a walk outside in the morning is the simplest fix. It also adds steps; see 7,000 vs 10,000 steps.
Blue light at night: real but overhyped
In a well-known lab study (Chang et al., 2015, PNAS), reading on a light-emitting e-reader before bed, compared with a printed book, delayed sleep onset, reduced melatonin, shifted the body clock later and left people less alert the next morning. That's a genuine effect.
Yet the market has run ahead of the science. A 2023 Cochrane review of blue-light filtering glasses found:
- Probably no meaningful benefit for eye strain from computer use in the short term.
- Very uncertain effects on sleep quality: of six small trials, three reported improvements and three found none.
- No evidence on macular health.
The broader lesson from the Brown recommendations is that total evening light matters, not just phones. Bright ceiling lights in the kitchen at 10 pm may matter as much as your screen. Night mode settings are harmless, but dimming the room and putting the phone away earlier are more reliable.
Light, sleep and recovery
Sleep is where much of training adaptation happens, as covered in VO2max and sleep. A stable light-dark pattern helps you fall asleep at a consistent time. If your HRV and resting heart rate worsen during dark winters or after late nights, light habits are one thing to look at.
Light exposure is also used to treat seasonal mood problems, and exercise has its own antidepressant effects; see VO2max and mental health. Seasonal or persistent low mood deserves professional help, not just a lamp.
Light and exercise timing
Light and training interact in a few ways, though evidence on performance is thin:
- Time of day changes performance. A 2020 review (Mirizio et al.) found short maximal efforts tend to peak in the late afternoon or early evening, but warm-ups and training regularly at one time reduce the gap.
- Exercise shifts the clock too. Youngstedt and colleagues (2019) found that one hour of moderate treadmill exercise advanced the clock when done at 7 am or between 1 and 4 pm, and delayed it when done between 7 and 10 pm. Morning daylight plus a morning session pushes in the same direction.
- Bright light before a race? One small study of eight men (Thompson et al., 2015) found evening bright light improved a 10 km time trial in the heat the next morning. Interesting, but far from a recommendation.
Practically: schedule hard sessions like 4×4 intervals when you feel sharpest and they don't disrupt sleep, take easy zone 2 runs outdoors in the morning, and track the long-term trend with the VO2max calculator.
Safety notes
- Never look directly at the sun. Morning light works through ambient daylight, not staring.
- Light therapy boxes can trigger mania in people with bipolar disorder; check with a doctor first.
- Shift workers need tailored plans; the general rules can backfire.
Light is cheap, natural and well studied for sleep. Different wavelengths are marketed for very different claims; see red light therapy for that evidence. Other home factors worth a sensible check include mold, water quality and whether you need a reverse osmosis filter.
Frequently asked questions
How much morning light do I need?
There is no exact prescription, but a 2022 expert consensus recommends at least 250 lux melanopic EDI at the eye throughout the day, which usually means time outdoors or near a large window. A morning walk is an easy way to get it.
Does blue light from phones ruin sleep?
Evening screen light can delay the body clock and make it harder to fall asleep, as shown in lab studies with e-readers. Content, alertness and late bedtimes matter too, and room lighting is often a bigger source than the phone.
Do blue-light blocking glasses work for sleep?
A 2023 Cochrane review found the evidence on sleep quality very uncertain, with mixed results across small trials. Dimming lights is cheaper and better supported.
Does light affect exercise timing?
Indirectly. Light sets your body clock, which influences when you perform best. Exercise itself also shifts the clock, with morning and early afternoon sessions advancing it and late evening sessions delaying it.
Sources
- Blume C, Garbazza C, Spitschan M. Effects of light on human circadian rhythms, sleep and mood. Somnologie, 2019.
- Brown TM, Brainard GC, Cajochen C, et al. Recommendations for daytime, evening, and nighttime indoor light exposure to best support physiology, sleep, and wakefulness in healthy adults. PLoS Biol, 2022.
- Chang AM, Aeschbach D, Duffy JF, Czeisler CA. Evening use of light-emitting eReaders negatively affects sleep, circadian timing, and next-morning alertness. Proc Natl Acad Sci USA, 2015.
- Singh S, Keller PR, Busija L, et al. Blue-light filtering spectacle lenses for visual performance, sleep, and macular health in adults. Cochrane Database Syst Rev, 2023.
- Youngstedt SD, Elliott JA, Kripke DF. Human circadian phase-response curves for exercise. J Physiol, 2019.
- Mirizio GG, Nunes RSM, Vargas DA, Foster C, Vieira E. Time-of-day effects on short-duration maximal exercise performance. Sci Rep, 2020.
- Thompson A, Jones H, Marqueze E, Gregson W, Atkinson G. The effects of evening bright light exposure on subsequent morning exercise performance. Int J Sports Med, 2015.