Light at night and modeled circadian disruption predict higher risk of mortality: A prospective study in >88,000 participants

Importance Light at night disrupts human circadian rhythms, which are critical for maintaining optimal health. Circadian disruption accompanies poor health outcomes that precede premature mortality, including cardiometabolic diseases. However, links between personal night light exposure and premature mortality risk have not been established. Objective To characterize the association of light at night with all-cause and cardiometabolic mortality risks and to understand the role of circadian disruption in these associations by applying a computational model of the response of the human circadian pacemaker to light. Design Prospective cohort study. Setting United Kingdom. Participants UK Biobank cohort, N=88,904, aged 62.4±7.8 years, 57% female. Exposure Participants wore activity tracking watches with light sensors for one week between 2013-2016. Twenty-four-hour light exposure profiles were extracted for each participant, and day-time and night-time hours were defined by factor analysis. A validated mathematical model of the human circadian pacemaker was applied to model circadian amplitude and phase from weekly light data. Main Outcome Cause-specific mortality (National Health Service) recorded in 2,605 participants across a mean (±SD) follow-up period of 6.31±0.83 years after light/activity tracking. Results Risk of all-cause mortality was higher in participants in the 90 th -100 th percentiles of night-light exposure (HR[95%CI]=1.30[1.15-1.48]), and for those between the 70 th -90 th percentiles (HR=1.16[1.04-1.28]), compared to the darkest 50%. Participants in the 90 th -100 th percentiles of night-light exposure also had higher risk of cardiometabolic mortality (HR=1.41[1.07-1.85]). Higher circadian amplitude predicted lower risks of all-cause mortality (HR = 0.94[0.91-0.97] per SD) and cardiometabolic mortality (HR=0.90[0.83-0.96]), and circadian phase that deviated from the group average predicted higher risks of all-cause mortality (HR=1.33[1.17-1.51]) and cardiometabolic mortality (HR=1.48[1.12-1.97]). These findings were robust to adjustment for age, sex, ethnicity, and sociodemographic and lifestyle factors. Conclusions and Relevance Minimizing exposure to light at night and keeping regular light-dark patterns that enhance circadian rhythms may promote cardiometabolic health and longevity. Key Points Question Is light exposure at night associated with risk of premature mortality? Findings Exposure to brighter light at night, recorded with personal light sensors in >88,000 participants, was associated with higher risk of mortality across a subsequent 6-year period. Computational modeling indicated that disrupted circadian rhythms may explain this higher mortality risk. Meaning Avoiding light at night may be a cost-effective and accessible recommendation for promoting health and longevity.