Epidemiologic studies of sleep disturbances and mood disorders that may provide more valid estimates of associations between these two conditions than clinical samples due to differential use of health care services. Increasing uniformity of questionnaires to assess sleep disturbances has decreased the variance in estimates of insomnia and hypersomnia within community samples. Women are more likely to report insomnia than men in every age group. There appear to be no clear racial or ethnic differences in rates of insomnia or hypersomnia. Several community-based studies have found that sleep disturbances are powerful risk factors for the development of new episodes of major depression in the following year. Individuals who report insomnia or poor quality sleep may be at higher risk for depression throughout their lifetime. Epidemiologic studies will be useful for developing the long-term perspective on the natural history of sleep disturbances and mood disorders and the consequences of treatment.