Although sleep disorders medicine is a relatively young discipline, understanding of the diagnosis, pathophysiology, and treatment of sleep disorders is evolving at a rapid pace. This overview discusses the history of the development of sleep disorders medicine, tracing changes in the diagnostic classification of sleep disorders as well as the role of polysomnography in diagnosis. This evolution is most evident for insomnia, one of the major sleep disturbances. The accumulation of epidemiologic data on the prevalence and temporal course of insomnia and emerging information regarding its pathophysiology derived from laboratory assessments have led to the development of new therapeutic approaches for primary insomnia and insomnia associated with medical and psychiatric disorders.