Going outdoors more during the day might lead to slipping into sleep more easily during the night, according to a new study that found a relationship between the seasons and shifts in sleeping patterns.
While the study found the number of hours of slumber banked across summer, fall, winter, and spring were similar, in the winter months, research participants went to sleep later in the evening and woke up later in the morning.
In line with earlier research, the team behind the new study thinks that less light exposure during the day can be responsible for shifts in our body clock that then mean we're less ready for sleep when night comes around.
"Our bodies have a natural circadian clock that tells us when to sleep at night," says biologist Horacio de la Iglesia from the University of Washington (UW) in Seattle.
"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 involved 507 UW undergraduate students, with data gathered between 2015 and 2018 through wrist monitors. The monitors measured both sleep activity and the participants' exposure to light.
During the winter, students went to bed an average of 35 minutes later and woke up an average of 27 minutes later than during the summer.
The numbers were somewhat surprising given Seattle's high latitude; there's plenty of natural daylight in the summer to make use of for evening activities.
The researchers suggest that light exposure has different impacts on the circadian clock at different times of the day. Light during the day is more likely to cause you to sleep earlier, while light at night – perhaps from artificial sources – will push back that timing, meaning your body isn't sleepy until later.
Data analysis revealed that each hour of daytime light exposure, even on cloudy days, shifted the students' circadian phases forward by around 30 minutes. However, each hour of evening light pushed these phases back by about 15 minutes.
"It's that push-and-pull effect," says de la Iglesia. "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."
This new study shows that light exposure during the day and night has an effect, too, with artificial light playing a role ever since the invention of the electric lightbulb. Our activities are no longer limited by the hours of daylight as they once were.
And the findings do, of course, go beyond students. No matter our age or way of life, more exposure to natural light during the day could be key to keeping our circadian phases more stable – and getting to bed at the same time each night.
"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.
"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 research has been published in the Journal of Pineal Research.