Brighter nights and darker days predict higher mortality risk: A prospective analysis of personal light exposure in >88,000 individuals

Abstract

Light enhances or disrupts circadian rhythms, depending on the timing of exposure. Circadian disruption contributes to poor health outcomes that increase mortality risk. Whether personal light exposure predicts mortality risk has not been established. We therefore investigated whether personal day and night light, and light patterns that disrupt circadian rhythms, predicted mortality risk. UK Biobank participants (N = 88,905, 62.4 ± 7.8 y, 57% female) wore light sensors for 1 wk. Day and night light exposures were defined by factor analysis of 24-h light profiles. A computational model of the human circadian pacemaker was applied to model circadian amplitude and phase from light data. Cause-specific mortality was recorded in 3,750 participants across a mean (±SD) follow-up period of 8.0 ± 1.0 y. Individuals with brighter day light had incrementally lower all-cause mortality risk (adjusted-HR 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 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. Day light, night light, and circadian amplitude predicted cardiometabolic mortality, with larger hazard ratios than for mortality by other causes. Findings were robust to adjustment for age, sex, ethnicity, photoperiod, and sociodemographic and lifestyle factors. Minimizing night light, maximizing day light, and keeping regular light-dark patterns that enhance circadian rhythms may promote cardiometabolic health and longevity.

Keywords: cardiometabolic; circadian disruption; light at night; light sensor; longevity.

Fig. 1.

Hazard ratios [95%CI] of all-cause, cardiometabolic, and other-cause mortality for light exposures across 24 h, including Model 1 (A) and Model 3 (B). Separate models were implemented for each half-hour clock time interval, with each model including 50 to 70%, 70 to 90%, and 90 to 100% light percentile groups referenced against 0 to 50%. Bonferroni correction for multiple comparisons required P < 0.001 for statistical significance. For example, in column one of Panel A, individuals in the 90 to 100th percentiles of light exposure at 04:00 had a significantly higher risk of all-cause mortality than those in the 0 to 50th percentiles (HR = 1.27), whereas individuals in the 50 to 70th percentiles and 70 to 90th percentiles at 04:00 did not have a significantly higher mortality risk. See SI Appendix, Fig. S2 for mortality risk by 24-h light exposures adjusted for Model 2 covariates.