Circadian rhythm disorders are disruptions in a person’s circadian rhythm-a name given to the "internal body clock" that regulates the (approximately) 24-hour cycle of biological processes in animals and plants. The term circadian comes from Latin words that literally mean "around the day."

The key feature of circadian rhythm disorders is a continuous or occasional disruption of sleep patterns. The disruption results from either a malfunction in the "internal body clock" or a mismatch between the "internal body clock" and the external environment regarding the timing and duration of sleep. As a result of the circadian mismatch, individuals with these disorders usually complain of insomnia at certain times and excessive sleepiness at other times of the day, resulting in work, school, or social impairment.

These are the more common circadian rhythm disorders:
Delayed Sleep Phase Disorder is a circadian rhythm disorder most common in adolescents and young adults whose "night owl" tendencies delay sleep onset -- often until 2 a.m. or later. If allowed to sleep in late (often as late as 3 p.m.), sleep deprivation does not occur. However, earlier wake up times can lead to daytime sleepiness and impaired work and school performance. These individuals are often perceived as lazy, unmotivated, or poor performers who are chronically tardy for morning obligations. People with delayed sleep phase syndrome are often most alert, productive, and creative late at night.

Advanced Sleep Phase Disorder is usually seen in the elderly. This disorder is identified by regular early evening bedtimes (6 p.m. - 9 p.m.) and early morning awakenings (2 a.m. - 5 a.m.). People with advanced sleep phase syndrome are "morning larks" and typically complain of early morning awakening or insomnia as well as sleepiness in the late afternoon or early evening.

Jet Lag results from a conflict between the pattern of sleep and wakefulness between the internal biological clock and that of a new time zone. Individuals find it hard to adjust and function optimally in the new time zone. Eastward travel is more difficult than westward travel because it is easier to delay sleep than to advance sleep.

Shift Work Disorder affects people who frequently rotate shifts or work at night. Work schedules conflicts with the body’s natural circadian rhythm and some individuals have difficulty adjusting to the change. Shift work disorder is identified by a constant or recurrent pattern of sleep interruption that results in insomnia or excessive sleepiness.