Light sets the body clock.

Why it matters

The body runs on an internal clock of roughly 24 hours. It times sleep, alertness, body temperature and the release of hormones such as melatonin. The clock drifts unless it is reset each day, and light reaching the eye is the main reset signal. Specialised light-sensitive cells in the retina, most responsive to blue-green light, carry that signal to the brain.

Buildings change the light we receive. Indoors, daytime light is usually far dimmer than outside, and electric light keeps evenings bright. When people camped with only natural light for a week, their internal night began close to sunset and ended near sunrise, noticeably earlier than at home ¹.

What the research shows

Evening light delays sleep. In a laboratory study, ordinary room light in the hours before bed suppressed melatonin and shortened its nightly duration by about 90 minutes compared with dim light ². Reading on a light-emitting tablet before bed, compared with a printed book, delayed sleep onset and the timing of the body clock and reduced alertness the next morning ³.

Daytime light supports sleep at night. Office workers who received more circadian-effective light in the morning fell asleep faster and reported better sleep and lower depression scores than those who received less ⁴. In a small pilot study, people working without windows reported poorer sleep quality and vitality than colleagues with more daylight ⁵.

Needs change with age. The lens of the eye lets through less short-wavelength light as we age and the pupil gets smaller, so older adults receive a weaker signal from the same room light ⁶.

There are now consensus targets. In 2022 an international expert group recommended, for healthy adults, bright light at eye level through the day, much dimmer light in the three hours before bed, and a dark sleeping environment. The targets are expressed as melanopic equivalent daylight illuminance (melanopic EDI), a measure of how strongly light acts on the body clock: at least 250 lux in the day, no more than 10 lux in the evening, and no more than 1 lux during sleep ⁷.

What can be done

Most improvements are about timing. Bring more daylight to the places people spend their mornings, and put desks and breakfast tables near windows. Use brighter, cooler-toned light for daytime tasks, and switch to warm, dim, low-placed light after dark. Dimmers and simple scene controls make this easy to live with. Bedrooms need darkness: effective window coverings and fewer glowing standby lights.

How we assess it

We measure light at eye level where people actually sit, stand and sleep, at the times of day they use each room, including melanopic EDI. We map daylight across the space, review fittings, lamps, colour temperature and controls, and check bedrooms at night.

Evidence at a glance

• Well supported

  Bright light in the evening delays the body clock and suppresses melatonin. ^2,3

• Well supported

  Daytime light exposure matters for sleep and alertness, and expert targets now exist. ⁷

• Reasonable

  Lighting that shifts colour and brightness through the day in homes. Plausible and low risk, but most trials are in offices, labs and care settings.

General information only, not medical advice. How we grade evidence.

References

1. Wright KP, McHill AW, Birks BR, et al. Entrainment of the human circadian clock to the natural light-dark cycle. Curr Biol. 2013;23(16):1554-8. doi.org/10.1016/j.cub.2013.06.039 · PubMed 23910656
2. Gooley JJ, Chamberlain K, Smith KA, et al. Exposure to room light before bedtime suppresses melatonin onset and shortens melatonin duration in humans. J Clin Endocrinol Metab. 2011;96(3):E463-72. doi.org/10.1210/jc.2010-2098 · PubMed 21193540
3. 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 U S A. 2015;112(4):1232-7. doi.org/10.1073/pnas.1418490112 · PubMed 25535358
4. Figueiro MG, Steverson B, Heerwagen J, et al. The impact of daytime light exposures on sleep and mood in office workers. Sleep Health. 2017;3(3):204-215. doi.org/10.1016/j.sleh.2017.03.005 · PubMed 28526259
5. Boubekri M, Cheung IN, Reid KJ, Wang CH, Zee PC. Impact of windows and daylight exposure on overall health and sleep quality of office workers: a case-control pilot study. J Clin Sleep Med. 2014;10(6):603-11. doi.org/10.5664/jcsm.3780 · PubMed 24932139
6. Turner PL, Mainster MA. Circadian photoreception: ageing and the eye's important role in systemic health. Br J Ophthalmol. 2008;92(11):1439-44. doi.org/10.1136/bjo.2008.141747 · PubMed 18757473
7. 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;20(3):e3001571. doi.org/10.1371/journal.pbio.3001571 · PubMed 35298459