Circadian Rhythm Abnormalities

Author's note, May 25 2006: I have copied the contents of this page to Wikipedia so that others can improve on it.

It is thought that DSPS is caused by an insufficient ability to reset the sleep/wake cycle in response to environmental time cues - perhaps the biological clocks of DSP individuals have an unusually long cycle, or perhaps they are not sensitive enough to time cues.

There are other circadian rhythm disorders:

Jet lag affects people who fly across time zones.
Shift workers who work at night often have trouble sleeping during the day.
There is an advanced sleep phase syndrome, which causes difficulty staying awake in the evening and staying asleep in the morning. It is essenally the reverse of DSPS.
Non 24-hour sleep-wake syndrome, also known as hypernychthemeral syndrome, causes patients to stay up later and later every night, then wake up later every morning. It occurs frequently among people who are totally blind. DSP individuals stay up late, but they can sleep at the same time every day. Non 24-hour sleep/wake syndrome is related to DSPS, only it is more severe and considerably more debilitating. It is believed to be rare.
Finally, an irregular sleep-wake pattern presents as sleeping at very irregular times, and usually more than once per day (waking frequently during the night and taking naps during the day).

DSPS has in some instances followed an illness or head injury, and might run in families. But in most cases, it is not known what causes the biological clock to be confused. Because DSPS was discovered only in 1981, and there is little awareness of the disorder, it is difficult to get a reliable estimate of how widespread DSPS is. About 7% of adolescents have DSPS, and it is probably responsible for 7 -10% of cases of chronic insomnia.

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