SleepShred
Get the app

Light: the world is physically brighter

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

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 · 6

The new world atlas of artificial night sky brightnessFalchi F, Cinzano P, Duriscoe D, et al. · 201680% of the world and 99% of US/European populations live under light-polluted skies — the 'world is physically brighter'.Artificially lit surface of Earth at night increasing in radiance and extentKyba CCM, Kuester T, Sánchez de Miguel A, et al. · 2017Satellite data: outdoor lit area and brightness grew ~2%/year 2012–2016.Exposure to room light before bedtime suppresses melatonin onset and shortens melatonin…Gooley JJ, Chamberlain K, Smith KA, et al. · 2011Ordinary room light in the hours before bed suppressed melatonin in 99% of people and shortened its duration ~90 min.Day and night light exposure are associated with psychiatric disordersBurns AC, Windred DP, Rutter MK, et al. · 2023Brighter days and darker nights were each independently linked to lower risk of depression and other disorders — daytime light offsets night light.Entrainment of the human circadian clock to the natural light-dark cycleWright KP Jr, McHill AW, Birks BR, Griffin BR, Rusterholz T, Chinoy ED · 2013A week of camping without electric light shifted the internal clock ~2 h earlier and aligned it to sunrise/sunset.Effects of light on human circadian rhythms, sleep and moodBlume C, Garbazza C, Spitschan M · 2019Accessible review of how timing, intensity and spectrum of light set the clock and affect sleep.

Related topics

Light

Frequently asked questions

Light: the world is physically brighter?

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

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.

Get SleepShred for iPhone

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.