PROJECT SUMMARY Circadian rhythms, generated by endogenous molecular clocks and entrained by the light-dark cycle, play a central role in orchestrating many physiological and behavioral processes related to cardiometabolic health. A growing body of evidence has linked light exposure at different times of the day and cardiometabolic outcomes, suggesting that characteristics of 24-hour light-dark cycle may be unique predictors of disease risk. However, there is a lack of investigation focusing on comprehensively assessing 24-hour light exposure, and in particular, its alignment with diurnal behavioral, to understand its health implications in human populations. A well- established hallmark of a healthy circadian timing system is the synchronization and proper alignment of multiple elements, such as between the environmental stimuli (e.g., light-dark cycle) and diurnal behaviors (e.g., activity- rest cycle). Our pilot study in older men applied phasor analysis to assess the alighnment between actigraphy- measured 24-hour light-dark and activity-rest cycles, and we found that impaired alignment between the two was associated with diabetes. These findings suggest that poor synchronization between the timing of light exposure and activity patterns may be a unique predictor of cardiometabolic risk, a novel hypothesis that needs to be tested in broader populations. We propose to test this hypothesis by conducting secondary data analysis in the National Health and Nutrition Examination Survey (NHANES, 2011-2014), which has detailed data on 24-hour light and activity patterns and rich information on cardiometabolic health. Moreover, the diverse and nationally representative sample will allow us to determine how 24-hour patterns of light exposure, its interrelationship with activity-rest cycles, and their associations with cardiometabolic outcomes may vary among subgroups with different demographic, socioeconomic, and occupational characteristics. Aim 1 Study characteristics of 24- hour light exposure and its alignment with rest and activity patterns in relation to cardiometabolic health in adult participants in NHANES. In this aim we will apply parametric and nonparametric methods to characterize multiple aspects of light-dark cycles and phasor analysis to characterize the alignment between light-dark and activity-rest cycles to study their associations with cardiometabolic outcomes. Aim 2 Study light exposure, alignment with rest and activity, and their associations with cardiometabolic health in adult populations with different demographic, socioeconomic and occupational attributes. In this aim we will examine multiple population attributes as determinants of 24-hour light-dark cycles and its alignment with activity- rest cycles, and as moderators of the association between light exposure and cardiometabolic health. Light exposure is a potentially modifiable risk factor for cardiometabolic diseases, and our study will help better understand the contribut...