The Effects of Seasonality on Sleep at the Polar Latitudes
At the polar latitudes, there are drastic changes in natural light levels between seasons. Light exposure has an influence on the circadian system, which can have a significant influence on sleep. Disruptions in the circadian system and sleep and short sleep duration have been reported to be associated with cardiac disease, obesity and diabetes among others. Because of this, our aim was to determine if sleep was affected when exposed to ~5.5 hours (winter solstice) verses 12 hours of light (spring equinox), while participants lived in a real world setting at 61° latitude. We hypothesized that sleep times would occur earlier during the equinox, when compared to the solstice, due to the natural light exposure entraining the circadian system to the 24h day.
METHODS:
To test this hypothesis, we conducted a within-subjects outpatient study (one week during the solstice and one week during the equinox). Eight participants (4 women) lived normally in their homes and had their sleep recorded through the use of call-ins to a time-stamped voicemails, sleep diaries and portable sleep monitors (measured individual sleep stages).
RESULTS:
During extended periods of daylight (equinox) participants received significantly less total sleep due to the significant earlier waketimes (p<0.05). Participants awakened earlier during weekdays when compared to weekends during both the equinox and solstice (p<0.05). Additionally, participants went to bed later on the weekend when compared to the weekdays during the solstice (p<0.05). There were no significant changes in sleep stages between the days of the week or seasons, however participants fell asleep significantly faster during the equinox (p<0.05), possibly due to the greater need for sleep, as sleep length was truncated during the equinox.
CONCLUSIONS:
Taken together, these results suggest that the seasonal changes of natural light impact sleep, which might have impacts on the underlying physiology, health and disease.