Circadian Rhythm Abnormalities
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.