Trouble falling asleep at night? Chase that daytime light, study shows

A study measuring the sleep patterns of students at the University of Washington has turned up some surprises about 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 Dec. 7 in the Journal of Pineal Research, the study found that UW students fell asleep later in the evening and woke up later in the morning during — of all seasons — winter, when daylight hours on the UW’s Seattle campus are limited and the skies are notoriously overcast.

The team behind this 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,” said senior author Horacio de la Iglesia, a UW professor of biology. “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 UW undergraduate students from 2015 to 2018. Data indicated that students were getting roughly the same amount of sleep each night regardless of season. But, on school days during the winter, students were going to bed on average 35 minutes later and waking up 27 minutes later than summer school days. This finding surprised the team, since Seattle — a high-latitude city — receives nearly 16 hours of sunlight on the summer solstice, with plenty evening light for social life, and just over eight hours of sunlight on the winter solstice.

“We were expecting that in the summer students would be up later due to all the light that’s available during that season,” said de la Iglesia.

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