Insomnia problems? It could depend on the (little) daylight

Insomnia problems? It could depend on the (little) daylight.

Published online in the Journal of Pineal Research, the study found that University of Washington students fell asleep later in the evening and woke later in the morning during the winter season, when daylight hours on the Seattle campus are limited and the sky is cloudy.


The team behind this study believe they have an explanation: Data has shown that in winter, students receive less exposure to light during the day. Other research has indicated that insufficient light during the day leads to problems at night, when it’s time to go to bed.

“Our body has a natural circadian clock that tells us when to go to sleep at night,” said lead author Horacio de la Iglesia, a biology professor. “If you don’t expose yourself to enough light during the day, when the sun is out , this ‘retards’ the clock and delays the onset of sleep at night.”

The study used wrist monitors to measure the sleep patterns and light exposure of 507 college students from 2015 to 2018. The data indicated that the students got roughly the same amount of sleep each night, regardless of season. However, on winter school days , students went to bed an average of 35 minutes later and woke up 27 minutes latercompared to summer school days. Based on the students’ sleep data, the researchers hypothesized that something in the winter “delayed” the students’ circadian cycles. For most humans, including college students, the innate circadian cycle is approximately 24 hours and 20 minutes and is “calibrated” daily by input from the environment.


Every hour of daylight shifts students’ circadian phases by 30 minutes . Exposure to outdoor light on overcast or overcast winter days in Seattle also had this effect, as it is still significantly brighter than indoor artificial lighting, de la Iglesia said. Each hour of evening light – that from indoor sources such as lamps and computer screens – delayed circadian phases by an average of 15 minutes. “It’s the push-and-pull effect,” de la Iglesia said. “And what we found is that because the students didn’t get enough daylight in the winter, their circadian clocks were delayed compared to the summer.”

“Many of us live in cities and towns with lots of artificial light and lifestyles that keep us indoors during the day,” de la Iglesia said. “This study shows that we have to go out, even for a short time and especially in the morning, to expose ourselves to natural lightIn the evenings, we minimize screen time and artificial lighting to help us fall asleep.”

  • Daytime light exposure is a strong predictor of seasonal variation in sleep and circadian timing of university students (

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