Hansen V, Skre I, Lund E.
Institute of Clinical Medicine, University of Tromsø, Norway.
Epidemiol Psichiatr Soc. 2008 Apr-Jun;17(2):120-7.

BACKGROUND: Seasonal Affective Disorder (SAD) is supposed to be caused by lack of daylight in winter. Yet the population of Northern Norway, living without sun for two winter months, does not spontaneously complain about depression during the dark period. 
AIMS: To summarize research bearing upon the validity of the concept of SAD.
METHOD: Review of relevant literature concerning the epidemiology of SAD and the questionnaire developed to measure it in general populations, the Seasonal Pattern Assessment Questionnaire (SPAQ).


RESULTS: Large population studies from northern Norway do not point to a higher prevalence of depression in winter than expected in any other general population. The psychometric properties of SPAQ are rather poor, and the diagnosis of SAD based on SPAQ bears little relationship to a meaningful concept of depression.
CONCLUSIONS: Seasonal Affective Disorder is not a valid medical construct. Instead, “Recurrent depression with seasonal pattern” as defined in the DSM-IV and the ICD-10 should be used as terms. However, more research is needed to establish whether it is at all fruitful to single out such a subtype of recurrent depression.