Circadian rhythm disorder: Disorders caused by the malfunctioning of the circadian system, or biological clock. A person's circadian rhythm is an internal biological clock that regulates a variety of body processes according to an approximate 24-hour period. All circadian rhythm disorders have in common the disruption of the normal sleep-wake cycle. Circadian rhythm disorders can be transient, such as jet lag, illness, and altered sleep cycle due to shift work. Chronic circadian rhythm disorders include delayed sleep-phase syndrome (DSPS), advanced sleep-phase syndrome (ASPS), and irregular sleep-wake cycle.
American Academy of Sleep Medicine. "Circadian Rhythm Disorders." 2008.