Lack of major seasonal variations in self reported sleep-wake rhythms and chronotypes among middle aged and older people at 69 degrees North: the Tromsø Study.

OBJECTIVE: Sleep timing and duration are influenced by outdoor light, but few studies have addressed these relationships in subarctic populations. We aimed to investigate how the extreme photic environment at 69 degrees north, with absence of daylight for 2months in the winter and constant light for 2months in the summer, would affect the sleep-wake rhythm.

METHODS: 4811 people ages 35-70years, from the cross sectional study 'Tromsø 6' responded to the Munich Chronotype Questionnaire, in addition to socio-demographic and health-related information. The seasonal distribution of chronotypes was calculated based on the MCTQ at the participation dates.

RESULTS: Late types had a 10min increase in sleep duration on free days from winter to spring, but had no seasonal variation in the average sleep duration; as well as no seasonal variations in mid sleep on free days was found. However, when chronotype distribution was adjusted for confounding factors e.g. age and sex, we found a significant advance in phase (8min) in the summer compared to the wintertime. This advance in MSFsc was significant both for the employed and the unemployed.

CONCLUSION: Other factors may be more important than daylight exposure in the regulation of sleep patterns for people in the subarctic. Moreover, the use of stimulants or excessive indoor and outdoor light may have masked the seasonal effect of variation in daylight. Further prospective studies and more research on clock-gene polymorphism, photosensitivity and other biological variables among subgroups of subarctic populations are needed.