The connection between mood and sleep disorders is highly complex and can be studied and interpreted in many respects. Epidemiologic data show that the co-occurrence of the two disorders is quite frequent. Thus an approach regarding them as a unit promotes biological psychiatric research by revealing new pathophysiological and therapeutic conclusions. Chronobiological results related to mood disorders have recently been described in excellent reviews including Hungarian ones. In the present review, the necessity of treatment of sleep disorders is evaluated in the context of relapse/remission/recurrence. Scientific data suggest that patients with insomnia have a ten-fold risk of developing depression, and insomnia plays an important role in depression relapses, recurrence of depressive episodes and becoming depression chronic. From neurobiological point of view, mood and sleep disorders have many features in common. Research has revealed decreased levels of melatonin and advanced sleep phases (shifted earlier) in depression, and altered and imbalanced monoaminergic pathways, and REM abnormalities in sleep disorders. Some authors suggest that REM abnormalities disappear along with the mood improvement, and the sleep structure can completely restore after remission. However, persistent abnormalities of REM sleep and slow wave sleep have also been found in remission, which increased the risk of the relapse and recurrence. Recently, there is an agreement as to the early treatment of insomnia can prevent the development of mood abnormalities. Alterations of cascades related to neural plasticity can also be a link between sleep and mood disorders. Neural plasticity is closely related to learning, sleeping, and cortisol regulation (coping with stress), and this draws the attention to comorbidity with further disorders (anxiety, dementia).