All patients must meet the diagnostic criteria for recurrent major depression or bipolar mood disorder. Seasonal affective disorder (SAD) is then a sub-type specifier used to describe temporal variations of these disorders. As such, SAD is not considered a stand-alone diagnosis or comorbid condition to recurrent major depression or bipolar disorder. Common presentations include the initiation or worsening of depressive symptoms during the autumn or winter months, and full remission during the spring or summer months, or hypo-manic or manic symptoms presenting during spring or summer months.
Circadian and neurotransmitter factors are likely to contribute to the pathophysiology of SAD, although the exact mechanism of action remains ill-understood. The suprachiasmatic nucleus (SCN) of the hypothalamus is being increasingly recognised as the 'master regulator' of several systems implicated in seasonal mood regulation. Diminished light during the autumn and winter may cause a phase shift in various circadian rhythms, including sleep-wake cycle, body temperature, hormone levels, and melatonin secretion.
Mortality estimates came from the national 1990–1994 mortality data base for the canton of Geneva, the most recent data available at the time of the study. Data were averaged over the 5-year period to avoid unstable mortality rates due to the small population (400 000). In Switzerland information on cause of death is coded according to the 8th Revision of the International Classification of Diseases (ICD-8), while DALY were developed using mainly the ICD-9 classification. Therefore coding was revised to ensure comparability.