The role of melatonin in seasonal affective disorder

Abstract

In humans, as in other mammals, melatonin is primarily a hormonal signal of darkness. To understand the role of melatonin in the winter form of seasonal affective disorder (SAD), it is important to note that melatonin is regulated by the photoperiod, exhibits a circadian rhythm and is a factor in regulating both circadian and seasonal rhythms.Winter depression, which is sensitive to the photoperiod, is in many respects analogous to the seasonal changes found in photosensitive animals. In those species changes in the duration of melatonin secretion, reflecting the changing photoperiod, are responsible for their seasonal reproductive, body, and behavioral changes. In winter SAD the current predominant theory is that a phase shift in circadian rhythms occurs so that there is an internal misalignment of circadian rhythms, which is responsible for the symptoms. Appropriately timed treatment with light, melatonin, or a combination of the two is currently the treatment of choice.Why patients with SAD are particularly prone to symptoms is not clear. Moreover, the role of melatonin in SAD is not well understood. It is known that both patients with winter depression and normal subjects can show a prolonged nocturnal melatonin rise in parallel with increased hours of darkness. Are patients with SAD like seasonal animals in being more sensitive to melatonin? These are major points that are dealt with in the present chapter.