The statement that circadian rhythmicity is an important component of mood regulation as well as a drive of mood disorders is supported by a growing body of evidence. Diurnal rhythms of the positive and negative components of mood as well as of the level of arousal depend on the circadian phase, the homeostatic sleep regulatory mechanisms and the harmonic interaction of the circadian and homeostatic processes. The chronopathological symptoms which are typical in depression and explain the blunted mood of depressive patients are of the phase-advance and phase-delay type characterized by a misalignment between the circadian rhythms and the sleep-wake schedules, best described by the phase-angle alterations. The abnormal phase angle between circadian rhythms and the timing of the sleep period could emerge from an interaction of the chronotypes and other constitutional factors with adverse environmental effects (inadequate zeitgebers) leading to a disharmony between the diurnal components of mood regulation and consequent extreme mood states. The aim of the chronotherapies of depression and of other affective disorders is that of resynchronizing the circadian rhythms or in other words to reconstitute the harmony between these subsystems. Pharmacological approaches, lifestyle changes and specific chronotherapeutic interventions might help to achieve this goal.