Light upon seasonality:seasonality of symptoms in the general population and in patients with depressive and anxiety disorders

The main question of this thesis is the extent to which the seasons influence positive and negative affect in humans. To answer this question, we analysed data in samples of the general population, in patient-oriented settings in primary care and specialised mental healthcare. This thesis shows that on a group level seasonal variation in depressive symptoms, positive and negative affect, is absent or limited in effect size. These findings hold for healthy individuals and for patients who suffer from a depressive disorder (with or without a comorbid anxiety disorder) but unexpectedly also for patients suffering from a seasonal affective disorder as measured with a questionnaire. Further, we found that as the degree of psychopathology increases, significantly more patients attribute their complaints to the change of the seasons. Also, patients suffering from seasonal affective disorder scored high on psychopathology and on a neuroticism scale. We showed that on a population level, seasonal variations in positive and negative affect and depressive symptoms are mainly attributable to high-neurotic subjects. From the literature and our studies, we infer that the psychological mechanism of cognitive attribution is an underestimated factor in seasonal affective disorder. The most important limitation of this thesis is the cross-sectional design of most studies, and as a consequence, the absence of longitudinal measurements within the same individual for all four seasons. We recommend initiating a study with a longitudinal repeated measures approach in order to assess the validity (and temporal stability) of the specifier “with seasonal pattern” for recurrent major depressive disorder and bipolar disorder