Daylight is an important zeitgeber for entraining the circadian rhythm to a 24 h clock cycle, especially within the Polar circle, which has long Polar nights several months each year. Phase delays in sleep timing may occur, but the mean shift is normally small. However, the individual variation in phase shifts is large, implicating moderating factors. Here we examined the role of several self-regulatory variables (mood and fatigue, behavioral habits, and psychological self-regulation) as moderators of seasonality in sleep timing and chronotype.
A sample of 162 young adults (76% females; mean age: females 23.4 years, males 24.3 years) participated in a prospective study across three seasons (September, December, March) in Tromsø/Norway at 69°39'N Sleep diary and sleep/health-related questionnaire data were collected at each time-point.
Sleep timing and chronotype were delayed during the dark period (December) compared with brighter photoperiods (September and March). Comparable effects were observed for insomnia, fatigue, mood (depression and anxiety), subjective health complaints, physical activity, and school-related stress. Most importantly, depression and fatigue moderated the degree of seasonal shifting in sleep timing, whereas the other self-regulation indicators did not (ie eating habits, physical activity, and psychological self-regulation).
Seasonality in sleep timing and chronotype was confirmed, and it seems that depressive symptoms during the dark period exacerbate phase-shifting problems for people living in sub-Arctic regions.