A study measuring students’ sleep patterns at the University of Washington has turned up some surprises about how seasons impact how and when our bodies tell us to sleep—and illustrates the importance of getting outside during the day, even when it’s cloudy.
Published online on Dec. 7 in the Journal of Pineal Research, the study found that University of Washington students fell asleep later in the evening and woke up later in the morning during—of all seasons—winter, when daylight hours on the Seattle campus are limited, and the skies are notoriously overcast.
The team behind the study believes it has an explanation: The data showed that in winter students received less light exposure during the day. Other research has indicated that getting insufficient light during the day leads to problems at night when it’s time for bed.
“Our bodies have a natural circadian clock that tells us when to go to sleep at night,” says senior author Horacio de la Iglesia, PhD, a University of Washington professor of biology, in a news release. “If you do not get enough exposure to light during the day when the sun is out, that ‘delays’ your clock and pushes back the onset of sleep at night.”
The study used wrist monitors to measure sleep patterns and light exposure for 507 University of Washington undergraduate students from 2015 to 2018. Data indicated that students were getting roughly the same amount of sleep each night regardless of the season. But, on winter school days, students went to bed on average 35 minutes later and woke up 27 minutes later than on summer school days.
“It’s that push-and-pull effect,” says de la Iglesia in the release. “And what we found here is that, since students weren’t getting enough daytime light exposure in the winter, their circadian clocks were delayed compared to summer.”
The study offers lessons not just for college students.
“Many of us live in cities and towns with lots of artificial light and lifestyles that keep us indoors during the day,” says de la Iglesia in the release. “What this study shows is that we need to get out—even for a little while and especially in the morning—to get that natural light exposure. In the evening, minimize screen time and artificial lighting to help us fall asleep.”