This article begins with a review of the major central nervous system functional systems that allow for optimal alertness during the waking day, and the rapid initiation and good maintenance of sleep at night. Subsequent sections discuss each of the 6 primary circadian rhythm sleep disorders. Attention is paid to known or suspected pathophysiology, diagnostic criteria and assessment methodology, and treatment options. The article concludes with a discussion of challenges that must be met to improve the recognition and treatment of these quite impactful sleep disorders.