Light

What It Is

The light around you during the day and after dark.

Why It Matters

Light is the main signal that sets your body clock, and the evidence is stronger for keeping nights dark than for making days bright. Among middle-aged and older UK adults who wore light sensors, those with the brightest night-time environments had higher all-cause mortality than those who slept in the dark, with the risk concentrated in the small hours. Read that as an association rather than a settled cause: re-analysing the same cohort with satellite estimates of outdoor night light, a much coarser stand-in for what reaches your eyes, found no such link once air pollution and evening noise were accounted for.

The mechanism is circadian. Melanopsin-containing cells in your retina report light to the body clock, and they respond at lower levels than most people expect: in one controlled study of young adults, evening light no brighter than ordinary indoor lighting was enough to suppress melatonin and push back its onset. Sensitivity varies enormously between people, so no single level suits everyone. The daytime half stands on weaker ground: brighter days predicted lower mortality, but much of that association disappeared in models accounting for physical activity, and being outdoors in daylight is hard to separate from being well enough to get outside.

Dim the lights for the last few hours, cover what glows in the bedroom, and get daylight early. Expert consensus sets the targets in melanopic equivalent daylight illuminance: under 1 lux while you sleep, under 10 in the three hours before bed, and at least 250 by day (a measure read at eye level and weighted to those retinal cells, so under warm household lighting it runs roughly a third to a half of a plain lux reading). That bar is reachable: on a plain lux reading, bedrooms typically measure around 1, with sleep disturbance more common above 3. Those targets are set for healthy adults aged 18 to 55 on regular daytime schedules, and reflect expert judgement rather than thresholds derived from health outcomes. Expect a modest return and mostly on timing: in pooled trials, largely in people with circadian rhythm disorders or psychiatric illness, light shifted delayed sleep earlier more clearly than it improved sleep continuity, where the benefit was small.

• Phillips, A.J.K. et al. (2019). High sensitivity and interindividual variability in the response of the human circadian system to evening light. Proceedings of the National Academy of Sciences. doi.org/10.1073/pnas.1901824116
• Faulkner, S.M. et al. (2019). Light therapies to improve sleep in intrinsic circadian rhythm sleep disorders and neuro-psychiatric illness: A systematic review and meta-analysis. Sleep Medicine Reviews. doi.org/10.1016/j.smrv.2019.04.012
• Brown, T.M. et al. (2022). Recommendations for daytime, evening, and nighttime indoor light exposure to best support physiology, sleep, and wakefulness in healthy adults. PLOS Biology. doi.org/10.1371/journal.pbio.3001571
• Obayashi, K. et al. (2022). Associations between indoor light pollution and unhealthy outcomes in 2,947 adults: Cross-sectional analysis in the HEIJO-KYO cohort. Environmental Research. doi.org/10.1016/j.envres.2022.114350
• Liang, X. et al. (2024). Outdoor light at night and mortality in the UK Biobank: a prospective cohort study. Occupational and Environmental Medicine. doi.org/10.1136/oemed-2023-109036
• 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
• Dai, J. et al. (2026). Objectively measured individual-level daytime vs. night-time light exposure and cardiovascular disease mortality in US adults. European Journal of Preventive Cardiology. doi.org/10.1093/eurjpc/zwag303