circadian- clock that is biological clock-

sleep disoderCircadian clock pertains to rhythmic biological cycles recurring at approximately 24-hour intervals.

The International Classification of Sleep Disorders (ICSD) is the most widely used classification system for sleep disorders. The third edition of the ICSD (ICSD-3) includes seven major categories of sleep disorders.


Sleep related breathing disorders

Central disorders of hypersomnolence

Circadian rhythm sleep-wake disorders


Sleep related movement disorders

Other sleep disorders

Circadian rhythm disorders are characterized by chronic or recurrent sleep disturbance due to misalignment between the environment and an individual’s sleep-wake cycle. There are several types of circadian rhythm disorders:

Shift work disorder and jet lag disorder are the most common types of circadian rhythm sleep disturbances, although these do not often come to medical attention.

Delayed sleep-wake phase disorder is characterized by sleep and wake times that are habitually delayed compared to conventional times. Sleep is usually normal once it is initiated. A typical patient has difficulty falling asleep and awakens late. This disorder is more prevalent in younger patients.

Advanced sleep-wake phase disorder is characterized by sleep and wake times that are habitually early compared to conventional time. A typical patient falls asleep early and awakens early spontaneously. This disorder is not commonly reported, but it is more likely to be observed in the elderly.

Irregular sleep-wake rhythm disorder is characterized by lack of a clearly defined circadian rhythm of sleep and wake. This disorder is commonly associated with developmental disorders in children and with neurodegenerative diseases such as Alzheimer disease, Parkinson disease, and Huntington disease.

Non-24-hour sleep-wake rhythm disorder is characterized by insomnia or excessive sleepiness that occurs because the intrinsic circadian pacemaker is not entrained to a 24-hour light/dark cycle. Most individuals with this disorder are totally blind, and the failure to entrain circadian rhythms is related to the lack of photic input to the circadian pacemaker.

Circadian rhythm disorders can also be due to a medical, psychiatric, or neurologic disorder.

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