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The term circadian rhythm sleep disorder describes a group of sleep disorders that affect an individual's circadian rhythm. The circadian rhythm is a cycle many organisms experience that lasts roughly the length of a day, 24 hours, and influences the timing of various activities, such as eating and sleeping. A circadian rhythm sleep disorder, then, is a disorder that affects an individual's ability to sleep and to stay awake. In essence, the "biological clock" is off and the person is not able to stay awake at times that are appropriate for work, class, or other important activities. Conversely, they are unable to get enough sleep to remain healthy and alert unless they sleep according to their biological clock.
One with a normal circadian rhythm is able to go to sleep and wake up at regular times, feeling well-rested. The body and mind become accustomed to sleeping and rising at a set time, and no internal or external factors prevent this. One is able to remain aware and alert throughout the day. A circadian rhythm sleep disorder prevents this healthy sleep cycle.
A circadian rhythm sleep disorder can be classified as either extrinsic or intrinsic, based on whether the source of the disorder is internal or external. People who work at times that conflict with their biological clock experience an extrinsic circadian rhythm sleep disorder, as do those who travel frequently and suffer the effects of jet lag as they cross time zones. Delayed sleep phase syndrome, or DSPS, is an intrinsic circadian rhythm sleep disorder that causes one to wake late and be particularly alert in the middle of the night. Advanced sleep phase syndrome causes the opposite response; an individual with the syndrome is unable to stay awake and alert in the evenings.
While there exists no catch-all cure for circadian rhythm sleep disorders, people suffering such disorders do have options. They can take medications, such as melatonin supplements, that are able to positively affect the circadian rhythm. Melatonin is a compound naturally developed in the human body and is involved in the regulation of the sleep cycle. People can also turn to behavioral therapy; generally they are told to avoid naps and caffeine until their cycles become more regular. Another option is bright light therapy, in which individuals are exposed to particularly bright light at certain times each day to influence their circadian rhythms, which can be somewhat light sensitive.