Improvement in mental health over time in northern Norway- a prospective study of a general population followed for 9 years, with special emphasis on the influence of darkness in winter.

Background: There are very few prospective studies of mental distress in the general population. Two studies of general health in Finnmark county in northern Norway, performed 9 years apart, contained questions about depression and sleeping problems, which made it possible to study change in mental distress over time in the general population.

Methods: In a self-administered questionnaire, the subjects were asked about depression and sleeping problems, both generally and related to the dark period in winter, at two points in time: in 1987/88 and 1996/97. The persons participating in the two surveys were partly the same and partly new individuals. More than 12000 persons responded in 1987/88 and more than 7000 people in 1996/97. The cohort that answered questions about sleeping problems and depression at both time-points consisted of 3318 and 3682 persons, respectively.

Results: The prevalence of depression and sleeping problems, both generally and related to winter,decreased significantly in the course of the 9-year time-span. The decrease was also true for the cohort that answered at both time-points, except for sleeping problems related to winter, which were unchanged. The pattern of changes was identical for both genders and all age groups.

Conclusions: Mental health in the general population seems to have improved during the period studied. The findings are also relevant for the discussion of the validity of the concept Seasonal Affective Disorder, and the presumed connection to light-deprivation.