Sleep disturbances are among the most common symptoms in patients who have acute episodes of mood disorders, and patients who have mood disorders exhibit higher rates of sleep disturbances than the general population, even during periods of remission. Insomnia and hypersomnia are associated with an increased risk for the development or recurrence of mood disorders and increased severity of psychiatric symptoms. Sleep electroencephalogram recordings have identified objective abnormalities associated with mood disorders, providing insight into the neurobiologic relationships between mood and sleep. Future studies will continue to investigate this association and potentially improve treatment of sleep and mood disorders.