# Light: the world is physically brighter

Skyglow, screens and 24-hour business — but morning light offsets evening light.



## Takeaways

- Get outdoor light in the morning (log the minutes).

- Dim the 2–3 hours before bed.

- Blackout / mask if urban.



Satellite data show the planet is physically brighter: 80% of the world lives under light-polluted skies and lit area grows ~2% a year. Skyglow (light bouncing off clouds), global business hours and urban living mean you're exposed to more light in the 3–4 hours before bed than any ancestor. Ordinary room light before bed suppresses melatonin in 99% of people. This is why blackout curtains and masks matter now when they didn't when you were a kid.

The good news: light exposure in the morning and throughout the day dramatically counteracts evening light. In >85,000 people, brighter days and darker nights were each independently protective. A week of camping realigns the clock ~2 h earlier. So an unavoidable bright walk at 10 pm does less damage if you saw the sun at 8 am.



## Sources

- The new world atlas of artificial night sky brightness (Falchi F, Cinzano P, Duriscoe D, et al., 2016)

- Artificially lit surface of Earth at night increasing in radiance and extent (Kyba CCM, Kuester T, Sánchez de Miguel A, et al., 2017)

- Exposure to room light before bedtime suppresses melatonin onset and shortens melatonin duration in humans (Gooley JJ, Chamberlain K, Smith KA, et al., 2011)

- 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)

- 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)

- Effects of light on human circadian rhythms, sleep and mood (Blume C, Garbazza C, Spitschan M, 2019)

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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.

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