Researchers have shed light on why winter can disrupt sleep for some people, a finding shared by the University of Washington press service.
The investigation looked at 507 students from the same university. The results showed that weekly sleep totals remained roughly steady across seasons. Yet on winter school days, students tended to go to bed about 35 minutes later and woke up around 27 minutes later than on summer days. The surprising pattern emerged in Seattle, a city situated far enough north to experience dramatic shifts in daylight across the year. During the summer solstice, residents enjoy roughly 16 hours of daylight, supporting after‑dark social life, while the winter solstice brings a little over eight hours of daylight. In summer, the extended daylight naturally encourages evening activity, a trend that, at first glance, seems logical.
The cause appears to lie in the body’s internal clock, known as the circadian rhythm. According to the researchers, daylight accelerates the clock during the day, especially in the morning, which helps people feel tired earlier in the evening. In contrast, light exposure late in the afternoon or early at night tends to delay fatigue by shifting the clock backward. The two effects collide, creating a shift in sleep timing that changes with the seasons. Each hour of the day nudges the clock forward by about 30 minutes, while evening time can push it forward by roughly 15 minutes. In the summer, the clock can fall behind because daylight is less intense in the morning and daylight exposure in the evening remains strong, affecting the overall sleep schedule.
Experts suggest spending more time outdoors in daylight during winter, because natural light — even on cloudy days — has a meaningful impact on the circadian system. This pragmatic approach helps align daily routines with the body’s internal timing, supporting healthier sleep patterns through the gray months.