A team of researchers led by the University of Washington (UW) has measured the sleep patterns of students and found that the participants fell asleep later in the evening and woke up later in the morning during winter, when daylight hours are limited and the skies are usually overcast. According to the experts, this phenomenon happens because in winter, people receive less light exposure during the day.
“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 professor of Biology at UW. “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 scientists used wrist monitors to examine the sleep patterns and light exposure of 507 UW undergraduate students from 2015 to 2018. While the analysis showed that students were getting about the same amount of sleep each night regardless of the season, on school days in winter, they were going to sleep on average 35 minutes later and waking up 27 minutes later than during summer – a rather surprising discovery.
“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. However, something in the winter appeared to be “pushing back” the students’ circadian cycles. For most humans, the innate circadian cycle governing when we are awake and when we sleep runs at approximately 24 hours and 20 minutes, and is calibrated by environmental inputs. This study revealed that the participants’ circadian cycles were running nearly 40 minutes later in winter than in summer.
While light appeared to play a major role in this phenomenon, the researchers stressed that light has different effects 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,” said de la Iglesia. “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.”
According to the scientists, while each hour of daytime light moved up the participants’ circadian phases by 30 minutes, each hour of artificial evening light delayed circadian phases by an average of 15 minutes.
“Many of us live in cities and towns with lots of artificial light and lifestyles that keep us indoors during the day. 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,” de la Iglesia advised.
The study is published in the Journal of Pineal Research.
By Andrei Ionescu, Earth.com Staff Writer
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