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Light: what the research actually says

Light is the main signal setting your clock: bright days and dark nights both count, and modern life inverts the pair.

All studies · 6

Day and night light exposure are associated with psychiatric disorders: an objective light study in >85,000 peopleBurns AC, Windred DP, Rutter MK, et al. · Nature Mental Health 1:853–862 · 2023Brighter days and darker nights were each independently linked to lower risk of depression and other disorders — daytime light offsets night light.Effects of light on human circadian rhythms, sleep and moodBlume C, Garbazza C, Spitschan M · Somnologie 23(3):147–156 · 2019Accessible review of how timing, intensity and spectrum of light set the clock and affect sleep.Artificially lit surface of Earth at night increasing in radiance and extentKyba CCM, Kuester T, Sánchez de Miguel A, et al. · Science Advances 3(11):e1701528 · 2017Satellite data: outdoor lit area and brightness grew ~2%/year 2012–2016.The new world atlas of artificial night sky brightnessFalchi F, Cinzano P, Duriscoe D, et al. · Science Advances 2(6):e1600377 · 201680% of the world and 99% of US/European populations live under light-polluted skies — the 'world is physically brighter'.Entrainment of the human circadian clock to the natural light-dark cycleWright KP Jr, McHill AW, Birks BR, Griffin BR, Rusterholz T, Chinoy ED · Current Biology 23(16):1554–1558 · 2013A week of camping without electric light shifted the internal clock ~2 h earlier and aligned it to sunrise/sunset.Exposure to room light before bedtime suppresses melatonin onset and shortens melatonin duration in humansGooley JJ, Chamberlain K, Smith KA, et al. · Journal of Clinical Endocrinology & Metabolism 96(3):E463–E472 · 2011Ordinary room light in the hours before bed suppressed melatonin in 99% of people and shortened its duration ~90 min.

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Frequently asked questions

What does the research say about light?

A week of camping without electric light shifted the internal clock ~2 h earlier and aligned it to sunrise/sunset.

How do I track light?

SleepShred logs it in about 20 seconds a day and then compares your nights with and without it, so you see the effect on your own sleep instead of the study average.

Is SleepShred free?

No. SleepShred Pro is $4.99 per month or $29.99 per year, with no free trial and no ads. Everything stays on your iPhone.

Track it, don't guess it

SleepShred turns this research into a 20-second daily habit: log your night, see which of your own behaviours move your sleep, and screen for the disorders nobody diagnoses.

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Educational summary of published research, not medical advice. Screening scores are not diagnoses. If a score is elevated or you feel unwell, talk to a clinician.