To sleep well, you have to go outside, even if the weather is gloomy. A new study from the University of Washington confirms the importance of daylight in the quality of sleep.
- A sleep cycle is based on the alternation of slow wave sleep and paradoxical sleep. A good night has between 3 to 6 successive cycles of 60 to 120 minutes each.
- According to the France Public Health Barometer on sleep published in 2019, adults sleep on average 6h42 per 24 hours on weekdays. This is below the recommended minimum daily 7 hours.
- 45% of 25-45 year olds believe they get less sleep than they need.
By launching their experiment on the influence of light on the circadian cycle, the researchers from the University of Washington (UW) thought they were highlighting that students go to bed later in the summer due to sunshine. Nevertheless, their work, published in the journal Journal of Pineal Research on December 7, 2022, show that in reality the sleep shift is greater in winter.
Going to bed and waking up later in winter
Scientists tracked the sleep of 507 students between 2015 and 2018 using monitors placed on their wrists. The data showed that the volunteers got roughly the same amount of sleep each night regardless of the season. On the other hand, they went to bed an average of 35 minutes later and woke up 27 minutes later on school days in the winter compared to the summer.
“This finding surprised the team since Seattle is a high-latitude city that receives nearly 16 hours of sunshine at the summer solstice, with plenty of evening light for socializing, and just over eight hours of daylight. ‘sunshine at the winter solstice’, specify the authors.
Faced with this element, the experts have hypothesized that this winter shift could come from the lack of light. “Our body has a natural circadian clock that tells us when to go to sleep at night. If you don’t get enough light exposure during the day when the sun is up, it ‘lags’ your clock and delays the onset of sleep at night. “explains Horacio de la Iglesia, professor of biology at UW and lead author.
Sleep: prefer morning light
The data collected showed that exposure to light during the day had a greater impact than in the evening. Each hour of daylight “went up” the pupils’ circadian phases by 30 minutes. This effect was also observed on cloudy days, as the brightness remains significantly brighter than artificial indoor lighting. Moreover, each hour spent in the evening with the lights of lamps and computer screens delayed the circadian cycle by 15 minutes.
“It’s a push-and-pull effect”, explained the professor of the Iglesia. “And what we found here was that since the students weren’t exposed to enough daylight in the winter, their circadian clock was delayed compared to the summer.”
He concludes : “this study shows that we need to go outside – even for a little while and especially in the morning – to have exposure to natural light. In the evening, minimize screen time and artificial lighting to help you fall asleep .”