A study measuring the sleep patterns of students at the 爱豆社区 has turned up some surprises about how and when our bodies tell us to sleep 鈥� and illustrates the importance of getting outside during the day, even when it鈥檚 cloudy.
Published online Dec. 7 in the Journal of Pineal Research, found that UW students fell asleep later in the evening and woke up later in the morning during 鈥� of all seasons 鈥� winter, when daylight hours on the UW鈥檚 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鈥檚 time for bed.
鈥淥ur bodies have a natural circadian clock that tells us when to go to sleep at night,鈥� said senior author , a UW professor of biology. 鈥淚f you do not get enough exposure to light during the day when the sun is out, that 鈥榙elays鈥� your clock and pushes back the onset of sleep at night.鈥�
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 鈥� a high-latitude city 鈥� 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.
鈥淲e were expecting that in the summer students would be up later due to all the light that鈥檚 available during that season,鈥� said de la Iglesia.
Based on student sleep data, the researchers hypothesized that something in winter was 鈥減ushing back鈥� the students鈥� circadian cycles. For most humans, including college students, the innate circadian cycle governing when we鈥檙e awake and asleep runs at about 24 hours and 20 minutes 鈥� and is 鈥渃alibrated鈥� daily by input from our environment. For UW students in the study, sleep data indicated that their circadian cycles were running up to 40 minutes later in winter compared to summer.
The team focused on light as a potential explanation for this winter delay. But light has different impacts on circadian rhythms at different times of the day.
鈥淟ight during the day 鈥� especially in the morning 鈥� advances your clock, so you get tired earlier in the evening, but light exposure late in the day or early night will delay your clock, pushing back the time that you will feel tired,鈥� said de la Iglesia. 鈥淯ltimately, the time that you fall asleep is a result of the push and pull between these opposite effects of light exposure at different times of the day.鈥�
Data showed that daytime light exposure had a greater impact than evening light exposure in the UW study. Each hour of daytime light 鈥渕oved up鈥� the students鈥� circadian phases by 30 minutes. Even outdoor light exposure on cloudy or overcast winter days in Seattle had this effect, since that light is still significantly brighter than artificial indoor lighting, said de la Iglesia. Each hour of evening light 鈥� light from indoor sources like lamps and computer screens 鈥� delayed circadian phases by an average of 15 minutes.
鈥淚t鈥檚 that push-and-pull effect,鈥� said de la Iglesia. 鈥淎nd what we found here is that since students weren鈥檛 getting enough daytime light exposure in the winter, their circadian clocks were delayed compared to summer.鈥�
The study offers lessons not just for college students.
鈥淢any of us live in cities and towns with lots of artificial light and lifestyles that keep us indoors during the day,鈥� said de la Iglesia. 鈥淲hat this study shows is that we need to get out 鈥� even for a little while and especially in the morning 鈥� to get that natural light exposure. In the evening, minimize screen time and artificial lighting to help us fall asleep.鈥�
Lead author on the paper is , an associate manager with the Allen Institute for Cell Science, who conducted the study as a UW doctoral student. Co-authors are UW undergraduate alum Isabelle Hua, now a researcher at the National Institute of Neurological Disorders and Stroke; Alex Grahe in the UW Department of Biology; Jason Fleischer and Satchidananda Panda of the Salk Institute; Kenneth Wright and C茅line Vetter of the University of Colorado, Boulder; and UW teaching professor of biology Jennifer Doherty. The research was funded by the National Science Foundation. Dunster was supported by the Riddiford-Truman Fellowship and the Hoag Endowed Graduate Fellowship through the UW Department of Biology.
For more information, contact de la Iglesia at horaciod@uw.edu.