Associations of latitude and photoperiod with sleep duration in a yearlong study of US physicians

Geographic factors may influence sleep, but prior studies have been inconsistent, likely because of reliance on seasonally invariant and self-reported data. Using year-long objective sleep measurements from 4683 U S. medical interns, this study aimed to determine how latitude affects total sleep time (TST) and assess whether photoperiod acts as a mediator in this association. Linear mixed models (LMMs) showed that each 1° increase in latitude was associated with approximately 0.23 min longer TST on average across the year, and daily photoperiod was negatively associated with TST. Simulation-based mediation analyses revealed seasonal effects: in summer, higher latitude led to reduced TST via longer photoperiod, whereas in winter, higher latitude led to increased TST via shorter photoperiod. Direct effects of latitude remained significant during winter, suggesting additional influences beyond photoperiod. These findings indicate that latitude influences TST through a seasonally specific photoperiod-mediated pathway, as well as through other non-photoperiod environmental factors.