SEATTLE — It’s a popular question among tired Americans: How can I sleep better without turning to medicine? The answer could be on the other side of your front door. Getting outside for at least a little while and soaking in some daytime light, even when it’s cloudy, can help us sleep soundly at night, new research from the University of Washington suggests.
Scientists measured the sleep patterns of UW undergrad students, observing that they tended to fall asleep especially late and wake up later in the morning during winter. Of course, that’s a time when when daylight hours on the UW Seattle campus are usually limited and the skies stay overcast.
Collected data confirmed that the observed students weren’t getting nearly as much daylight during winter in comparison to other seasons. Study authors theorize this is why they slept less. Numerous earlier studies have indicated that lack of daylight exposure can lead to restlessness come bedtime.
“Our bodies have a natural circadian clock that tells us when to go to sleep at night,” says senior author Horacio de la Iglesia, a UW professor of biology, in a statement. “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.”
Wrist monitors were used to gauge both sleep patterns and light exposure among 507 undergrad students between 2015 and 2018. That data indicated students were getting about the same amount of sleep daily regardless of the season. However, on school days during the winter, participants went to bed an average of 35 minutes later and woke up 27 minutes later than summer school days. Study authors admit they were surprised by their findings, especially considering Seattle is a high-latitude city that enjoys close to 16 hours of sunlight on the summer solstice, over eight hours of sunlight on the winter solstice, and plenty of year-round evening light and nightlife.
“We were expecting that in the summer students would be up later due to all the light that’s available during that season,” Prof. de la Iglesia adds.
Based the collected data, study authors posit that something was happening during winter to “push back” the students’ circadian cycles. The instinctual circadian cycle governing when we wake up and sleep runs at about 24 hours and 20 minutes for most people, but is also “calibrated” on a daily basis by environmental stimuli. The students’ circadian cycles were running up to 40 minutes later in winter than summer.
How daylight impacts our sleep more than evening light
Researchers focused on light as a potential catalyst for these findings. It’s important to note, however, that light can have a number of different impacts on circadian rhythms at different times of the day. “Light during the day — especially in the morning — 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,” Prof. de la Iglesia explains. “Ultimately, 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.”
The data indicates that daytime light exposure has a bigger impact on sleep than evening light exposure. More specifically, each additional hour of daytime light “moved up” the students’ circadian phases by a half hour. Even if it was cloudy or overcast during the winter, outdoor light exposure still had this effect. Each hour of evening light (light derived from indoor sources like lamps and computer screens) meanwhile, delayed circadian phases by about 15 minutes. Cloudy daylight is still significantly brighter than artificial indoor lighting.
“It’s that push-and-pull effect,” Prof. de la Iglesia comments. “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.”
In conclusion, researchers say these findings may prove valuable to far more people than just college students.
“Many of us live in cities and towns with lots of artificial light and lifestyles that keep us indoors during the day,” Prof. de la Iglesia concludes. “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.”
The study is published in the Journal of Pineal Research.