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 out during the day, even when it’s cloudy.
Daytime light exposure is a strong predictor of seasonal variation in college students’ sleep and circadian rhythm
Summary
In the absence of electric light, human sleep usually begins shortly after dark and at higher latitudes, Daily sleep time changes seasonally as the photoperiod changes.However, access to electric light protects humans from the natural changes of photoperiod, and it remains controversial whether seasonal changes in sleep occur despite this isolation from the natural cycle of light and dark. We measured sleep time in more than 500 college students living in the city of Seattle, WA (47.6°N) throughout the four seasons; We show that even when students follow a school schedule, 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 (on daylight saving time) than students during summer school days, a change that is one hour higher relative to solar midnight.In addition, the chronotype defined by half sleep on days off corrected for excess sleep (MSFc), an indirect estimate of the circadian phase, was more than 30 minutes later in the winter compared to summer.
Analysis of the effect of light exposure showed that the number of hours of light exposure 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 brought forward 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. In addition, the time of day of exposure to high light intensities was more predictive of MSFc when daytime exposure was considered than when exposure was considered during the full 24 hours of the day.Our results show that although sleep time is highly synchronized with social time, a delay in sleep time during the winter months is evident. They also suggest that daily exposure to daylight is key to preventing this delayed phase of the circadian clock and thus the disruption of the circadian rhythm that is usually exacerbated in high-latitude winters.
Figure 1 Hours of sleep and light exposure during the school day by season. 24-hour sleep clock graphs and light exposure on school days. Sleep and light data were aggregated throughout each season (n = 507; Autumn = 138, Winter = 88, Spring = 183, Summer = 98). Gray portions indicate night with average sunrise and sunset times, yellow slices representing solar noon and midnight at clock time. During the fall daylight saving time ended in the United States. Hashed segments indicate solar noon and midnight during Standard Time (ST) (full graphs 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 average clock time for sleep onset (purple), the end (light blue), the first and last exposures to 5 (dark blue), 50 (green), and 500 ( rosa) lux. See Table 1 and Supporting Information: Table S3 to obtain statistics on each variable.
Source – https://www.intramed.net/102806