• seasonal affective disorder;
  • epidemiology;
  • Norway

In an epidemiological study carried out in the town of Tromsö in northern Norway an extensive questionnaire was sent twice, in summer and winter, to 1000 subjects to investigate the degree of symptoms of mood variation (depression or hypomania) during the polar winter night and midnight sun seasons. The results suggest that the general population north of the Arctic Circle shows a major seasonal mood variation. It is suggested that seasonal mood variation is a chronobiological mood disturbance related to and probably precipitated by extreme variations in light. The therapeutic implications and preventive measures are discussed.