0358 Seasonal Rhythms in Wearable-Derived Sleep Across U.S. Latitudes

Abstract Introduction Humans exhibit circadian rhythms in sleep, but it is unclear whether they also display annual cyclical patterns. In other species, circannual rhythms often vary with latitude, with greater amplitude and predictable shifts in peak timing at higher latitudes. Demonstrating similar latitudinal clines in human sleep would provide evidence consistent with a circannual component to annual sleep variation. This study tests whether daily total sleep time (TST) and sleep midpoint exhibit annual rhythmicity and whether the amplitude and timing of these rhythms vary across geographic location. Methods This study uses yearlong wearable sleep data from the 2018–2022 cohorts of the Intern Health Study (IHS), a longitudinal study of first-year medical residents. Annual seasonality in daily sleep was tested using cosinor models. Latitude effects were evaluated in mixed-effects models that allowed the amplitude and phase of these rhythms to vary by location. Amplitude and peak timing were derived from model coefficients for interpretation. Results Both TST and sleep midpoint showed evidence of annual rhythmicity. Average TST did not differ significantly by latitude; however, peak TST was slightly greater and occurred earlier at higher latitudes. In contrast, neither sleep midpoint amplitude nor phase varied significantly across latitudes. Conclusion Annual rhythms in both TST and sleep midpoint, along with modest latitudinal differences in TST amplitude and peak timing, support the presence of latitudinal clines in annual sleep patterns, consistent with a circannual component for duration but not timing. Future research could explore whether these annual patterns reflect endogenous circannual processes, environmental influences, or their interaction by incorporating objective light exposure and physiological circadian markers to more directly assess mechanisms underlying annual sleep variation. Support (if any)