Abstract Body: Introduction: Light is the strongest zeitgeber (time cue) for the human central circadian clock, and its effects on health may vary depending on the time of day by either strengthening or dampening circadian rhythms. However, the relations between various light exposures and human health remain largely unknown, partly due to the lack of available measurements.
Hypothesis: We hypothesized that greater nighttime and evening light exposure would be associated with a higher mortality risk, while greater daytime light exposure would be associated with a lower mortality risk.
Methods: This population-based prospective cohort study included 7,909 participants aged ≥20 years, with ≥4 days of data from wrist-worn accelerometers with light sensors measuring ambient light (lux) at 1 Hz from the U.S National Health and Nutrition Examination Survey 2011-2014. Mortality data through 2019 were obtained via linkage to the National Death Index. The average intensity of ambient light exposure during daytime (6:00 AM-7:59 PM), evening (8:00-11:59 PM), and nighttime (0:00-5:59 AM) were calculated. Cox models were used to estimate hazard ratios (HRs) for mortality.
Results: During a median (IQR) follow-up of 6.8 (5.7-7.8) years, 803 deaths were documented, including 255 from cardiovascular disease and 192 from cancer. The multivariable-adjusted HRs (95% CIs) for all-cause mortality were 0.82 (0.66-1.03) for the second quartile of daytime light exposure, 0.83 (0.67-1.04) for the third quartile, and 0.67 (0.51-0. 89) for the highest quartile, compared to the lowest quartile (P for trend <0.001). Conversely, there was no association between nighttime (P for trend = 0.14; HR for the highest vs lowest quartile: 0.94 [0.75-1.19]) or evening light exposure (P for trend = 0.46; HR for the highest vs lowest quartile: 0.85 [0.71-1.03]) and all-cause mortality risk. Similar inverse associations of daytime light exposure were observed for cardiovascular disease mortality (P for trend = 0.04; HR for highest vs lowest quartile: 0.65 [0.36-1.20]) but not for cancer mortality (P for trend = 0.27; HR for highest vs lowest quartile: 0.89 [0.54-1.48]).
Conclusions: In this nationally representative sample of U.S. adults, greater objectively measured ambient light exposure during daytime was inversely associated with all-cause and cardiovascular disease mortality risk. These findings may have significant public health implications given the reduced exposure to daytime light in modern society.