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.