# 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

- **Day and night light exposure are associated with psychiatric disorders: an objective light study in >85,000 people** — Burns AC, Windred DP, Rutter MK, et al. (2023), Nature Mental Health 1:853–862. Brighter 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 mood** — Blume C, Garbazza C, Spitschan M (2019), Somnologie 23(3):147–156. Accessible 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 extent** — Kyba CCM, Kuester T, Sánchez de Miguel A, et al. (2017), Science Advances 3(11):e1701528. Satellite data: outdoor lit area and brightness grew ~2%/year 2012–2016.

- **The new world atlas of artificial night sky brightness** — Falchi F, Cinzano P, Duriscoe D, et al. (2016), Science Advances 2(6):e1600377. 80% 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 cycle** — Wright KP Jr, McHill AW, Birks BR, Griffin BR, Rusterholz T, Chinoy ED (2013), Current Biology 23(16):1554–1558. A 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 humans** — Gooley JJ, Chamberlain K, Smith KA, et al. (2011), Journal of Clinical Endocrinology & Metabolism 96(3):E463–E472. Ordinary room light in the hours before bed suppressed melatonin in 99% of people and shortened its duration ~90 min.

---
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.

SleepShred for iPhone: https://apps.apple.com/app/id6802040541