Light at night
Your body clock is far more sensitive to evening and night-time light than most people assume: for the average young adult, a few evening hours of ordinary living-room light can halve melatonin. In a large study of people who wore light sensors on their wrists, the brightest nights tracked with higher death, diabetes and heart-disease rates, and the brightest days with lower death rates. The evidence is observational and comes from one research group with lighting-industry ties, but the fix is cheap and has no downside.
Get bright light by day, dim and warm the last three hours before bed, and sleep in near darkness. A dimmer and warm bulbs are better evidenced than blue-blocking glasses.
How little light it takes
Light is the strongest setter of the body clock: bright morning light moves it earlier, bright evening light pushes it later and suppresses melatonin, the hormone that signals night. The threshold is low. In a study of 55 adults aged 18 to 30 exposed to light for five evening hours, the average person lost half their melatonin at only about 25 lux — dimmer than most living rooms. People varied more than fiftyfold, from about 6 lux for the most sensitive to about 350 for the least.[1]
The expert targets
A 2022 expert consensus set targets in a unit weighted to the light-sensitive cells in the eye that drive the clock (melanopic equivalent daylight illuminance, measured at eye level): at least 250 lux of this melanopic light during the day, under 10 in the three hours before bed, and under 1 during sleep.[2] Real homes fall well short: modelling of domestic lighting predicted that about 48% of homes would suppress melatonin by at least half in the evening.[3]
Bright nights, dark days and hard outcomes
The most direct evidence comes from about 89,000 UK adults who wore a wrist light sensor for a week. Over roughly eight years, those with the brightest nights had 21–34% higher mortality than those with the darkest, while those with the brightest days had lower mortality. Deaths from heart and metabolic disease showed the strongest links, and a weaker daily rhythm independently predicted death.[4]
The same group of people has been followed for other outcomes:
- Type 2 diabetes. In about 85,000 adults over nearly eight years, the brightest-night group had a 53% higher risk of developing type 2 diabetes, rising step by step with light exposure and independent of genetic risk.[5]
- Heart disease. In about 89,000 adults averaging 62 years old, comparing the brightest nights with the darkest, heart failure was about half again as common and heart attacks about 50% more common, with coronary disease, atrial fibrillation and stroke all up by a quarter to a third — after adjusting for physical activity, smoking, diet, sleep duration and inherited risk.[6]
Why to hold these numbers loosely
All three studies are observational and come from a single sensor subsample of the same cohort, studied by the same research group. Two senior authors on these papers co-direct a circadian-lighting company and have received funding from lighting firms. People with brighter nights may differ in ways the adjustments miss, and poor health may itself bring more night-time light rather than the reverse.
Outdoor night light, measured by satellite, lost its link with death once air pollution and noise were accounted for. What seems to matter is the light you are actually exposed to, not the street outside. The practical message holds regardless: minimise night light, maximise day light.
Blue-blocking glasses don't earn their reputation
A Cochrane review found six small randomised trials, in 148 people, pointing in different directions, at very low certainty.[7] A 2025 analysis that measured sleep with wrist trackers rather than questionnaires found no effect on falling asleep or anything else — though on only three trials and 49 people, so a weak null rather than proof they do nothing.[8] Dimming the room is simpler, better evidenced and free. And blue-light filters on screens during the day cut exactly the light your clock needs.
What to do
- Get outdoor light by day, especially within an hour of waking.
- Dim and warm the last three hours before bed: warm bulbs (about 2700 K or lower), dimmers, and no overhead lights late in the evening.
- Make the bedroom near-dark for sleep, with blackout curtains if outside light gets in.
- Skip blue-blocking glasses as your main fix, and don't use blue-light filters in the daytime.
See Circadian rhythms for how light fits with sleep timing and meals.
Further reading
- Windred DP et al. Brighter nights and darker days predict higher mortality risk. PNAS 2024.[9]
- Windred DP et al. Personal light exposure patterns and incidence of type 2 diabetes. Lancet Reg Health Eur 2024.[10]
- Windred DP et al. Light Exposure at Night and Cardiovascular Disease Incidence. JAMA Netw Open 2025.[11]
- Phillips AJK et al. High sensitivity and interindividual variability in the response of the human circadian system to evening light. PNAS 2019.[12]
- Brown TM et al. Recommendations for daytime, evening, and nighttime indoor light exposure. PLOS Biology 2022.[13]
- Singh S et al. Blue-light filtering spectacle lenses for visual performance, sleep, and macular health in adults. Cochrane Database Syst Rev 2023.[14]