Sleep problems are an essential part of the current diagnostic criteria for depressive and bipolar disorders in children and adolescents. Whereas many studies have reported subjective sleep problems in youth with depression or bipolar disorder, except for reduced rapid eye movement latency associated with depression, few objective mood-related sleep abnormalities have been consistently identified. Recent technologic advances, such as spectral EEG and actigraphy, hold promise for revealing additional objective disturbances. There are presently few evidence-based published practice recommendations for mood-related sleep problems in youth. In this article, the authors chronologically review research on the phenomenology and treatment of sleep difficulties in youth with depressive and bipolar disorders and present research-based and clinically guided recommendations for the assessment and treatment of these problems.