Sleep research in depressive disorders has advanced considerably in the last two decades. In this review we highlight the different facets of this research, including neurophysiological descriptions of sleep disturbances in depression and data regarding the effects of antidepressant treatments on sleep. The available information is discussed taking into consideration a possible pathogenic role of certain sleep changes in depression, as assumed by several theoretical models. The scope of these models and their explanatory power are briefly analysed. Some comments are made on conceptual and methodological problems in the field of sleep research in psychiatric disorders.