Circadian rhythms are endogenous, near-24-hour biological rhythms that exist in all living organisms. The internal near-24-hour circadian clock is entrained or synchronized to the 24-hour light-dark cycle. Circadian rhythm sleep-wake disorder (CRSWD) is caused by alterations of the circadian time-keeping system, its entrainment mechanisms, or a misalignment of the endogenous circadian rhythm and the external environment.

The most common presenting symptoms of CRSWDs are difficulty initiating and maintaining sleep, and excessive sleepiness, but their impact extends to adverse health outcomes, impairments in social, occupational and educational performance, and safety concerns.