Brighter Nights and Darker Days Predict Higher Mortality Risk

A prospective cohort study suggests that personal exposure to brighter nights and darker days causes circadian disruption with poor health outcomes and increased mortality risk. Avoiding night light and seeking daylight may promote optimal health and longevity. From 2013 to 2016, 88,905 UK Biobank participants (mean age 62.4 years, 56.9% female, 97.0% white) wore light sensors for 1 week. Cause-specific mortality was recorded in 3,750 participants with 8 years of follow-up. A computational model of the human circadian pacemaker was applied to model circadian amplitude and phase from light data. Individuals with brighter daylight (07:30-20:30) had incrementally lower all-cause mortality risk (adjusted hazard ratio [aHR] ranges: 0.84 to 0.90 [50 to 70th light exposure percentiles], 0.74 to 0.84 [70 to 90th], and 0.66 to 0.83 [90 to 100th]), and those with brighter night light (00:30-06:00) had incrementally higher all-cause mortality risk (aHR ranges: 1.15 to 1.18 [70 to 90th], and 1.21 to 1.34 [90 to 100th]), compared to individuals in darker environments (0 to 50th percentiles). Individuals with lower circadian amplitude (aHR range: 0.90 to 0.96 per SD), earlier circadian phase (aHR range: 1.16 to 1.30), or later circadian phase (aHR range: 1.13 to 1.20) had higher all-cause mortality risks. Daylight, night light, and circadian amplitude predicted cardiometabolic mortality, with higher HRs than for mortality by other causes. Findings were robust to adjustment for age, sex, ethnicity, photoperiod, and sociodemographic and lifestyle factors. Short sleep partially explains the higher mortality risk observed in people with brighter night light exposure, but not long or fragmented sleep. The findings are consistent with previous cohort studies demonstrating higher all-cause mortality risk for people living in areas with brighter nights (satellite-derived), and for people with self-reported lower daylight exposure. Light enhances or disrupts circadian rhythms, depending on the timing of exposure. Light in the early and late biological night delays and advances circadian phase, respectively, and light near the middle of the biological night suppresses circadian amplitude. Circadian disruption contributes to poor health outcomes that increase mortality risk. Minimizing night light, maximizing day light, and keeping regular light–dark patterns that enhance circadian rhythms may promote cardiometabolic health and longevity. Source: https://www.pnas.org/