--- topic: "Light at Night" title: "Light at Night and Your Health" order: 1 description: "Bedroom light and bright nights are linked to worse sleep, metabolism and mood. What a night of room light does in the lab, who is affected, and cheap fixes." question: "Does sleeping with lights on or living with night-time light harm your health?" answer: "Likely, in bright bedrooms: one night of room light raised insulin resistance in a lab, and cohorts link night light to obesity and depression." actions: - "Darken the bedroom with blackout blinds, taped-over LEDs or a sleep mask: the expert target is under 1 melanopic lux [5], and masks raised melatonin in simulated hospital nights [12]." - "Dim the lights for the last 2–3 hours before bed (consensus: start at least 3 hours before [5]); even 10–50 lux delayed melatonin in lab tests [6]." - "Turn off the TV and keep any night-light low and out of view: sleeping with a TV or light on went with higher obesity risk in 43,722 women [9]." cautions: - "Night-shift workers: IARC classes night shift work as probably carcinogenic (Group 2A), so discuss schedule and light strategies with your clinician [11]." - "Keep safety first: if anyone gets up at night, use a dim path light rather than total darkness; the consensus allows up to 10 melanopic lux for night-time tasks [5]." confidence: 3 confidenceNote: "A small randomised lab trial and several large cohorts agree in direction, but the cohort links are observational and the trial was one night in 20 people." evidenceSummary: rctCount: 1 observationalCount: 4 metaAnalysisCount: 1 totalParticipants: "About 130,000 across four cohorts" replicated: true newestStudyYear: 2024 references: - id: "1" pmid: "35286195" title: "Light exposure during sleep impairs cardiometabolic function." authors: "Mason IC, Grimaldi D, Reid KJ, et al." year: 2022 journal: "Proc Natl Acad Sci U S A" - id: "2" pmid: "23118419" title: "Exposure to light at night, nocturnal urinary melatonin excretion, and obesity/dyslipidemia in the elderly: a cross-sectional analysis of the HEIJO-KYO study." authors: "Obayashi K, Saeki K, Iwamoto J, et al." year: 2013 journal: "J Clin Endocrinol Metab" - id: "3" pmid: "28992236" title: "Bedroom Light Exposure at Night and the Incidence of Depressive Symptoms: A Longitudinal Study of the HEIJO-KYO Cohort." authors: "Obayashi K, Saeki K, Kurumatani N" year: 2018 journal: "Am J Epidemiol" - id: "4" pmid: "39070751" title: "Personal light exposure patterns and incidence of type 2 diabetes: analysis of 13 million hours of light sensor data and 670,000 person-years of prospective observation." authors: "Windred DP, Burns AC, Rutter MK, et al." year: 2024 journal: "Lancet Reg Health Eur" - id: "5" pmid: "35298459" title: "Recommendations for daytime, evening, and nighttime indoor light exposure to best support physiology, sleep, and wakefulness in healthy adults." authors: "Brown TM, Brainard GC, Cajochen C, et al." year: 2022 journal: "PLoS Biol" - id: "6" pmid: "31138694" title: "High sensitivity and interindividual variability in the response of the human circadian system to evening light." authors: "Phillips AJK, Vidafar P, Burns AC, et al." year: 2019 journal: "Proc Natl Acad Sci U S A" - id: "7" pmid: "21193540" title: "Exposure to room light before bedtime suppresses melatonin onset and shortens melatonin duration in humans." authors: "Gooley JJ, Chamberlain K, Smith KA, et al." year: 2011 journal: "J Clin Endocrinol Metab" - id: "8" pmid: "24210607" title: "Let there be no light: the effect of bedside light on sleep quality and background electroencephalographic rhythms." authors: "Cho JR, Joo EY, Koo DL, et al." year: 2013 journal: "Sleep Med" - id: "9" pmid: "31180469" title: "Association of Exposure to Artificial Light at Night While Sleeping With Risk of Obesity in Women." authors: "Park YM, White AJ, Jackson CL, et al." year: 2019 journal: "JAMA Intern Med" - id: "10" pmid: "36228789" title: "Association between exposure to light at night (LAN) and sleep problems: A systematic review and meta-analysis of observational studies." authors: "Xu YX, Zhang JH, Tao FB, et al." year: 2023 journal: "Sci Total Environ" - id: "11" url: "https://www.iarc.who.int/news-events/iarc-monographs-volume-124-night-shift-work/" title: "IARC Monographs Volume 124: Night Shift Work" authors: "International Agency for Research on Cancer" year: 2020 journal: "IARC" - id: "12" pmid: "20398302" title: "Effects of earplugs and eye masks on nocturnal sleep, melatonin and cortisol in a simulated intensive care unit environment." authors: "Hu RF, Jiang XY, Zeng YM, et al." year: 2010 journal: "Crit Care" --- Likely, in bright bedrooms. In a small lab trial, one night of moderate room light (100 lux) during sleep raised heart rate and made the body less responsive to insulin the next morning [1]. Large studies link brighter bedrooms and nights with obesity, depression and type 2 diabetes [2][3][4]. Those links are observational, so not proof, but a dark bedroom is cheap and the experts' target [5]. ## How it works Your brain's clock reads light through the eyes and uses it to time sleep, hormones and metabolism. Light at the wrong time sends the wrong message. **How little it takes.** In 55 young adults, evening light of 10, 30 and 50 lux delayed the melatonin rise by 22, 77 and 109 minutes. Half the maximum effect came at about 25 lux, and people differed more than tenfold in sensitivity [6]. In 116 volunteers, ordinary room light before bed delayed melatonin onset in 99% and shortened melatonin duration by about 90 minutes compared with dim light [7]. For screens specifically, see [Evening Screen Exposure](/blue-light/evening-screen-exposure). **Light during sleep.** In a randomised trial, 20 healthy adults spent a night in dim light or in 100 lux of room light. Room light raised heart rate, lowered heart rate variability and raised next-morning insulin resistance [1]. In a small lab study, ten sleepers slept more lightly, with more arousals, when the lights were on [8]. **In real bedrooms.** Among 528 older Japanese adults, bedrooms averaging 3 lux or more went with higher body weight, waist size and triglycerides, and with obesity (odds ratio 1.89) [2]. Among 863 adults free of depression, those with bedrooms of 5 lux or more had 1.89 times the risk of developing depressive symptoms over about two years [3]. In 43,722 US women, sleeping with a light or television on went with a 33% higher risk of becoming obese [9]. In 84,790 people wearing light sensors, the brightest tenth of nights went with a 53% higher risk of type 2 diabetes [4]. **Streetlight and shift work.** A meta-analysis of 7 cross-sectional studies (577,932 people) linked light at night to 22% greater odds of sleep problems, with indoor light mattering more than outdoor light [10]. IARC classes night shift work as "probably carcinogenic" (Group 2A) [11]. **Practical targets.** An expert consensus recommends light no brighter than 10 'melanopic' lux (a measure of how strongly light drives the body clock) for at least 3 hours before bed, and a sleep room as dark as possible, under 1 melanopic lux [5]. In a simulated hospital night, eye masks with earplugs improved sleep and raised melatonin [12]. For other habits, see [Sleep Hygiene](/sleep/sleep-hygiene). ## The evidence in detail **Randomised lab trial.** Mason and colleagues randomised 20 healthy adults to a second night in dim light (under 3 lux, n = 10) or a second night under overhead room light at 100 lux (n = 10), after a dim first night. After the room-light night, HOMA-IR, a measure of insulin resistance, rose about 15% against a 4% fall in the dim group (P = 0.018). Heart rate was higher, heart rate variability lower, and slow-wave and REM sleep took up a smaller share of the night. A higher sympathetic share of heart rhythm correlated with a larger early insulin response (R = 0.65 across both groups). Glucose stayed normal because insulin rose to compensate. It was a single night in healthy, normal-weight volunteers, so lasting effects are unknown [1]. **Sleep depth.** In Cho and colleagues' polysomnography study, ten healthy sleepers slept once with the lights off and once with them on. Lights-on sleep had more light stage 1 sleep, less slow-wave sleep and a higher arousal index, with lower slow-oscillation, delta and spindle power [8]. **Evening and sleep-hours light.** Gooley and colleagues analysed 116 volunteers aged 18 to 30 living in a research centre. Compared with dim light (under 3 lux), room light (under 200 lux) in the 8 hours before bed delayed melatonin onset in 99.0% of people and shortened melatonin duration by about 90 minutes. Room light during the usual hours of sleep suppressed melatonin by more than 50% in 85% of trials [7]. Phillips and colleagues exposed 55 people to 10 to 2,000 lux for 5 evening hours. The light level giving half the maximal melatonin suppression was 24.6 lux for the group, but ranged from 6 lux in the most sensitive person to 350 lux in the least [6]. **Cohorts.** In the HEIJO-KYO study, bedroom light was measured every minute for two nights. Of 528 adults (mean age 72.8), the 145 sleeping at an average of 3 lux or more had higher BMI (23.3 vs 22.7), waist (84.9 vs 82.8 cm) and triglycerides (119.7 vs 99.5 mg/dl), and lower HDL cholesterol, than the 383 in dimmer rooms. Odds ratios were 1.89 for obesity by BMI and 1.72 for dyslipidemia, independent of urinary melatonin [2]. A follow-up of 863 adults without depressive symptoms (median 24 months) found that 73 developed them. Those in bedrooms averaging 5 lux or more (153 people) had a hazard ratio of 1.89 (1.13 to 3.14), or 1.72 (1.03 to 2.89) after adjusting for hypertension, diabetes and sleep [3]. In the Sister Study, 43,722 women free of cancer and cardiovascular disease were followed for 5.7 years on average. Sleeping with a television or light on in the room went with a relative risk of 1.33 (1.13 to 1.57) for incident obesity and 1.17 (1.08 to 1.27) for gaining 5 kg or more, compared with no light. Light exposure was self-reported [9]. In UK Biobank, 84,790 people wore light sensors for a week and were followed for about 8 years, with 1,997 new cases of type 2 diabetes. Compared with the darkest half of nights, hazard ratios were 1.29 (1.14 to 1.46), 1.39 (1.24 to 1.57) and 1.53 (1.32 to 1.77) across progressively brighter night-light groups, independent of genetic risk [4]. **Meta-analysis, outdoor light and shift work.** A meta-analysis of 7 cross-sectional studies (577,932 people) found higher light at night went with 22% greater odds of sleep problems (summary odds ratio 1.22, 1.13 to 1.33), more strongly for indoor light (1.74) than outdoor light (1.19) [10]. IARC classified night shift work as "probably carcinogenic" on limited human evidence for breast, prostate, colon and rectal cancer, sufficient animal evidence and strong mechanistic evidence in experimental animals [11]. **Null and conflicting findings.** Not every result lines up. Urinary melatonin did not differ between the bright-bedroom and dim-bedroom groups, so melatonin suppression may not explain the metabolic links [2]. In the lab trial a single night of light changed insulin handling but not glucose [1]. The outdoor-light association is smaller than the indoor one, and every study in that meta-analysis was cross-sectional [10]. We found no randomised trial showing that darkening bedrooms improves long-term health. **Practical guidance and its limits.** The recommendations of Brown and colleagues are an expert consensus, not trial results: in daytime at least 250 melanopic lux at the eye, from 3 hours before bed no more than 10, and in the sleep room no more than 1 (10 for night tasks). Melanopic lux counts how strongly light drives the clock cells in the eye, and it differs from the reading on a phone light meter [5]. In 14 healthy volunteers in a simulated intensive care unit, earplugs and eye masks gave more REM sleep, less arousal and higher melatonin [12]. **Why caution is warranted.** The cohort links are modest and observational. People who sleep with the TV on, sleep badly or are depressed may differ in other ways, and reverse causation is possible, because low mood or insomnia may lead people to leave lights on. The lab trial was small and short. **Bottom line (confidence 3).** Bright bedrooms and bright nights likely impair sleep and metabolic health and are linked to mood, with a plausible mechanism and a consistent direction across designs. The size of the real-world effects is uncertain. Darkening the bedroom is cheap and low-risk.