Chronobiology of Mood Disorders

The strong intimate relationship between mood disorders and life rhythms has been nowadays clearly recognized and analyzed, becoming one of the cornerstones for pathophysiological theorizations and therapeutic interventions in these diseases. Several body functions undergo biological rhythm, e.g., daily variations in hormone secretion or in body temperature or the sleep-wake cycle. Endogenous rhythms are primarily regulated by the circadian clock, a cluster of nerve cells that have their main localization in the hypothalamic suprachiasmatic nucleus. Endogenous rhythms may in turn be modulated by multiple exogenous clues, first of all the dark/light daily variations. Endogenous and exogenous rhythms intermingle in very complex associations, whose effects on human behavior, as well as their molecular determinants, are becoming to be elucidated. The term chronobiology refers both to the characterization of the biological underpinnings of life rhythms and to the clarification of their effects on several biological functions, including behavioral disease. The scope of this chapter is to provide an appraisal of the newest reports on the chronobiology of mood disorders. We will first describe the structural and functional anatomy, as well as the genetic, of the circadian clock, i.e., the suprachiasmatic nucleus. Then, we will review recent findings on the neurobiology and neuroimaging of the sleep-wake cycle. In the third part of the chapter, we will deal with the neurobiology of stress and its relationship with circadian rhythms. In the last section of the chapter, the biological effects of circadian rhythms and stress on affective states and mood disorders will be summarized and discussed.