The role of sunlight in sleep regulation: analysis of morning, evening and late exposure - BMC Public Health

Background Recent lifestyle changes have reduced sunlight exposure, impacting circadian rhythms and sleep regulation. This study investigates how sunlight exposure at different times of the day affects sleep parameters. Methods This cross-sectional study included 1,762 adults from the Iron Quadrilateral region, Brazil, and was conducted between October and December 2020. Sunlight exposure was self-reported for three periods: before 10 a.m., between 10 a.m. and 3 p.m., and after 3 p.m. Sleep quality was assessed using the Pittsburgh Sleep Quality Index (PSQI), which also provided information on total sleep time (total minutes slept), sleep latency (time to fall asleep), sleep efficiency (ratio of time spent asleep to time in bed), and the midpoint of sleep (the halfway point between sleep onset and wake-up time, indicative of circadian rhythm alignment). Associations between sunlight exposure and sleep outcomes were evaluated using linear regression models, adjusted for sociodemographic and behavioral variables. Results The midpoint of sleep was the most affected sleep parameter, showing significant associations with sunlight exposure across all timeframes, particularly in the morning. Every 30-minute increment of morning sun exposure (before 10 a.m.) was associated with a 23-minute reduction in the midpoint of sleep (-0:23 hh: mm; 95%CI: -0:36, -0:10; beta: -0.387; 95%CI: -0.607, -0.166). Sunlight exposure after 3 p.m. also reduced the midpoint of sleep but to a lesser extent (-0:19 hh: mm; 95%CI: -0:36, -0:03; beta: -0.325; 95%CI: -0.600, -0.051). Furthermore, an increase in morning sun exposure was significantly associated with a lower PSQI total score, improving sleep quality (beta: -0.184; 95%CI: -0.362, -0.006). No significant associations were observed between sunlight exposure and total sleep time, latency, or efficiency. Conclusion Morning sunlight exposure influences the regulation of the sleep midpoint and overall sleep quality. These findings highlight the potential role of morning sun exposure in aligning circadian rhythms and improving sleep health.