is a major health problem in the Untied States and one of the most common psychiatric illnesses. Some studies estimate that up to 40% of people seeing primary care physicians
exhibit some depressive symptoms, and these range in severity from infrequent periods of "blues" all the way up to patterns of behavior consistent with major depressive disorder. It has been observed that in a subpopulation of depressed patients, their propensity for depressive episodes varies seasonally. The term applied to this pattern is Seasonal Affective Disorder (SAD). The clinical definition of SAD is a pattern of recurring depressive episodes with seasonal onset and remission. Most patients experience worsening of their symptoms in the fall or winter. However, interestingly, there is a variant of the disorder with worsening of depressive episodes occurring in the spring, but this is distinctly less common. SAD is more prevalent at higher latitudes where there is more seasonal variation in climate and less light during winter months. The true prevalence of SAD is hard to determine due to overlap with other disorders, however the prevalence has been estimated between 1% and 9% of the population in northern climates. The pathogenesis of the disorder is incompletely understood, but several theories exist.
It is generally believed that decreasing daylight in winter months is central the genesis of the disorder, but no studies have definitively proven this to be true. Some hypotheses point to alterations in the circadian rhythm and abnormal melatonin metabolism as part of the link between light levels and mood in patients with SAD. Other theories involve the effect of environmental light levels on the level of the neurotransmitter serotonin produced in the brain. Although the biological mechanism of the disease is unclear, clinical studies have validated the SAD is indeed a real and treatable disorder. Any individual with seasonal "blues" or other symptoms of depression should visit a psychiatrist
as soon as possible for a full evaluation. Depression and SAD are serious but treatable disorders and it is advisable to be evaluated in person by a psychiatrist as soon as possible to avoid the substantial morbidity and even mortality associated with these disorders.