Alteration of the sleep-wake cycle and of the sleep structure are core symptoms of a major depressive episode, and occur both in course of bipolar disorder and of major depressive disorder. Many other circadian rhythms, such as the daily profiles of body temperature, cortisol, thyrotropin, prolactin, growth hormone, melatonin and excretion of various metabolites in the urine, are disrupted in depressed patients, both unipolar and bipolar individuals. These disrupted rhythms seem to return to normality with patient recovery. Research on circadian rhythms and sleep have led to the definition of nonpharmacological therapies of mood disorder that can be used in everyday practice. These strategies, named chronotherapeutics, are based on controlled exposures to environmental stimuli that act on biological rhythms, and demonstrate good efficacy in the treatment of illness episodes. They include manipulations of the sleep-wake rhythm (such as partial and total sleep deprivation, and sleep phase advance) and of the exposure to the light-dark cycle (light therapy and dark therapy). In recent years, an increasing literature about the safety and efficacy of chronobiological treatments in everyday psychiatric settings has supported the inclusion of these techniques among the first-line antidepressant strategies for patients affected by mood disorders.