Seasonal vulnerability to depression. Implications for etiology and treatment

The risk for depression increases at two opposite times of the year--late spring/early summer and late fall/early winter. In 15% of patients with recurrent major depression, depressive episodes regularly recur on an annual basis in one of the two seasonal risk periods. Thus, there are primarily two forms of seasonal affective disorder: recurrent fall-winter depression and recurrent spring-summer depression. The opposite seasonal types of depression tend to have opposite vegetative symptoms. Sleep, appetite and weight increase in winter depression and decrease in summer depression. An important implication of the seasonality of depression is that some type of depression may be caused by changes in the physical environment and that manipulations of the physical environment may be used as treatments. There is now extensive evidence that exposure to bright artificial light is an effective treatment of recurrent winter depression. A corollary is that seasonal deficiency of natural light probably induces winter depression. There have been considerable efforts to elucidate the biological mechanisms of winter depression and its response to phototherapy. Although no single system has been shown to be responsible for the syndrome, there is evidence that the indole hormone melatonin, the indole neurotransmitter serotonin, and the peptide neurohormone corticotropin releasing hormone (CRH) play roles in the pathophysiology and phototherapy of winter depression.