Abstract
Background and Aim: Light exposure is the most powerful resetting signal for circadian rhythms. Environmental circadian misalignment (ECM) occurs from geographic variation in light exposure due to location within a time zone (delayed circadian phase from less morning and greater evening light exposure moving west in a time zone). The aim of this study was to develop and validate the first-ever nationwide spatiotemporal exposure model of ECM in the US. Methods: For all US census tracts, the ECM exposure model incorporated elevation (in meters) and the average difference (in minutes) in sunrise time and sunset time between the census tract center of population and the eastern-most time zone boundary. Higher ECM exposure values indicate higher circadian misalignment due to higher elevation (fewer obstructions/extended field of view) and larger differences in sunrise and sunset times. Spearman correlation coefficients and multivariable linear regression adjusting for age, sex, race, ethnicity, median household income, and population density were used to validate the ECM exposure model with census tract-level prevalence of obesity (2016-2019) (known outcome associated with circadian misalignment). Results : Among the 60,943 census tracts included in this study, obesity prevalence was 32.62%. ECM exposure was positively correlated with obesity prevalence, with the strongest correlation in the Eastern time zone (rs=0.44, p<0.01). The highest vs. lowest ECM quintile was associated with higher obesity prevalence in the Eastern (adjusted β=6.00, 95% CI 6.68, 7.11) and Central time zones (adjusted β=2.85, 95% CI 2.62, 3.09), which are the US time zones characterized by the largest east-to-west distance (i.e., opportunity for ECM). Conclusions: We developed and validated a US nationwide spatiotemporal ECM exposure model, which can be used for exposure assessment in epidemiologic studies to enable comprehensive characterization of geographic variation in light exposure potentially impacting circadian phase. Keywords: light exposure; circadian misalignment; exposure model
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