A study that measured the sleep patterns of University of Washington students revealed some surprises about how and when our bodies tell us to sleep, and illustrates the importance of getting outside during the day, even when it’s cloudy.
Daytime light exposure is a strong predictor of seasonal variation in sleep and circadian rhythm of college students Summary In the absence of electric light, human sleep generally begins shortly after dusk and at higher latitudes, daily sleep time changes seasonally as the photoperiod changes. However, access to electric light protects humans from natural photoperiod changes, and it remains controversial whether seasonal changes in sleep occur despite this isolation from the natural light-dark cycle. We measured sleep time in more than 500 college students living in the city of Seattle, WA (47.6°N) across four seasons; we show that even when students follow a school schedule, the sleep schedule is delayed during the fall and winter. For example, during winter school days, students fell asleep 35 minutes later and woke up 27 minutes later (in daylight saving time) than students during summer school days, a change that is one hour longer in relationship with solar midnight. Furthermore, the chronotype defined by half sleep on days off corrected for excess sleep (MSFc), an indirect estimate of circadian phase, was more than 30 minutes later in winter compared to summer. Analysis of the effect of light exposure showed that the number of hours of exposure to light of at least 50 lux during the day was a stronger predictor of MSFc than the time of exposure to this illuminance after dark. Specifically, MSFc was advanced by 30 min for each additional hour of light exposure during the day and delayed only 15 min for each additional hour of light exposure after dark. Furthermore, time of day of exposure to high light intensities was more predictive of MSFc when daytime exposure was considered than when exposure during the full 24 hours of the day was considered. Our results show that although sleep time is highly synchronized with social time, a delay in sleep time is evident during the winter months. They also suggest that daily daylight exposure is key to preventing this delayed phase of the circadian clock and therefore circadian rhythm disruption that is typically exacerbated in high-latitude winters. |
Figure 1 Hours of sleep and light exposure during the school day by season. Clock graphs of 24 hours of sleep and light exposure on school days. Sleep and light data were aggregated across each season (n = 507; Fall = 138, Winter = 88, Spring = 183, Summer = 98). The gray slices indicate the night with average sunrise and sunset times, the yellow slices represent solar noon and midnight in clock time. During the fall, daylight saving time ended in the United States. The hashed segments indicate solar noon and midnight during standard time (ST) (full plots with DST and ST can be found in Supporting Information: Figure S1). The width of each color box represents the interquartile range with the black lines representing the mean clock time for sleep onset (purple), sleep end (light blue), first and last exposure to 5 (dark blue). , 50 (green) and 500 ( pink) lux. See Table 1 and Supporting Information: Table S3 for statistics on each variable.
Comments
While humans have managed to separate themselves from the natural world, they cannot escape it completely.
Published in the Journal of Pineal Research , the study found that UW students fell asleep later at night and woke up later in the morning during, of all seasons, winter, when daylight hours in The UW Seattle campus are limited and skies are noticeably cloudy.
The team behind this study believes they have an explanation: the data showed that in winter students received less light exposure during the day. Other research has indicated that insufficient lighting during the day causes problems at night, when it is time to go to bed.
“Our bodies have a natural circadian clock that tells us when to go to sleep at night,” said lead author Horacio de la Iglesia, a UW biology professor. "If you don’t get enough light exposure during the day when the sun is out, that ’sets back’ your clock and delays the onset of sleep at night."
The study used wrist monitors to measure the sleep patterns and light exposure of 507 UW undergraduates between 2015 and 2018. The data indicated that the students slept about the same number of hours each night, regardless of the season. . But on winter school days, students went to bed on average 35 minutes later and woke up 27 minutes later than on summer school days. This finding surprised the team, since Seattle, a high-latitude city, receives almost 16 hours of sunlight on the summer solstice, with plenty of nightlight for social life and just over eight hours of sunlight on the summer solstice. winter.
“We expected that in the summer the students would wake up later because of all the light available during that season,” de la Iglesia said.
Based on the students’ sleep data, the researchers hypothesized that something about the winter was "delaying" the students’ circadian cycles. For most humans, including college students, the innate circadian cycle that governs when we are awake and asleep lasts approximately 24 hours and 20 minutes, and is "calibrated" daily with information from our environment. For the 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 possible explanation for this winter delay. But light has different impacts on circadian rhythms at different times of the day.
“Light during the day, especially in the morning, sets your clock forward, so you get tired earlier in the evening, but exposure to light late in the day or early at night will set your clock back, causing you to feel tired.” He said of the Church. "Ultimately, when you fall asleep is a result of the push and pull between these opposing effects of light exposure at different times of the day."
The data showed that daylight exposure had a greater impact than nighttime light exposure in the UW study. Each hour of daylight "increased" the students’ circadian phases by 30 minutes. Even exposure to outdoor light on cloudy or overcast winter days in Seattle had this effect, as that light is still significantly brighter than artificial indoor lighting, de la Iglesia said. Each hour of evening light (light from indoor sources such as lamps and computer screens) delayed circadian phases by an average of 15 minutes.
“It’s that push-pull effect,” he said of the Church. "And what we found here is that because the students were not exposed to enough daylight in the winter, their circadian clocks were delayed compared to the summer."
The studio offers lessons not only for university students.
“Many of us live in cities and towns with a lot of artificial light and lifestyles that keep us indoors during the day,” de la Iglesia said. “What this study shows is that we need to go outside, even for a little while and especially in the morning, to get that exposure to natural light. At night, minimize screen time and artificial lighting to help us fall asleep.”
Conclusions
Our results suggest that even in highly urbanized conditions, daylight exposure may be more important in determining sleep timing than exposure to electric light at night.
They also highlight the importance of recommending not only avoidance of bright electric light at night, but also exposure to natural light during the day , to counteract the adverse effects of a delayed circadian phase during the winter months.38 This recommendation may be particularly relevant at relatively high latitudes that are also characterized by overcast winter skies, as is the case in Seattle.
Our results support a general influence of the natural environment on human circadian rhythms even in highly urbanized conditions, and are in line with our recently published study showing that sleep timing in the same university students studied here is synchronized with the lunar phase. Unlike sunlight, the changes associated with the lunar phase are much less prominent signals to the average individual living in post-industrial conditions. The fact that these subtle environmental cues can have an impact on human behavior supports the conclusion that while humans have managed to separate themselves from the natural world, they cannot escape it completely .
