Light for Steadier Sleep

What It Is

Getting daylight by day and keeping the bedroom dark at night, to make your sleep more regular.

Why It Matters

Light keeps your body clock in step with the 24-hour day, and daylight can help hold its timing steady, which is where the familiar advice of bright days and dark nights comes from.¹ Boddy grades this advice on regularity, how steady your sleep timing is from night to night, and the evidence is very uncertain about whether changing your light makes your sleep more regular. Nothing we have found is strong enough to size an effect on regularity, so there is no gain we could ask you to expect.

For long-term health, the signal is clearer for nights than for days. Among middle-aged and older UK adults who wore light sensors for a week, those with the brightest nights had higher all-cause mortality than those who slept in the dark, with cardiometabolic risk peaking for light in the small hours.¹ Read that as an association rather than a settled cause: its authors note that unmeasured differences between people could explain it.¹ The daytime half is weaker: brighter days still predicted lower mortality, but less so once physical activity, smoking, alcohol, urban living, and social life were accounted for, and time in daylight comes bundled with being physically active.¹

None of this says light does not matter. A dark bedroom and time outdoors are cheap habits, and reasonable ones to keep. What the evidence cannot support is a promise that they will steady your sleep, which is why this sits here rather than as advice under Regularity. What the regularity target itself asks for is a steady bedtime and wake time.

• 1 Windred, D.P. et al. (2024). Brighter nights and darker days predict higher mortality risk: A prospective analysis of personal light exposure in >88,000 individuals. Proceedings of the National Academy of Sciences. doi.org/10.1073/pnas.2405924121