Circadian rhythm sleep disorders are a family of sleep disorders affecting, among other things, the timing of sleep. People with circadian rhythm sleep disorders are unable to sleep and wake at the times required for normal work, school, and social needs. They are generally able to get enough sleep if allowed to sleep and wake at the times dictated by their body clocks. Unless they have another sleep disorder, their sleep is of normal quality.
Humans, like most animals and plants, have biological rhythms, known as circadian rhythms, which are controlled by a biological clock and work on a daily time scale. These affect body temperature, alertness, appetite, hormone secretion etc. as well as sleep timing. Due to the circadian clock, sleepiness does not continuously increase as time passes. A person's desire and ability to fall asleep is influenced by both the length of time since the person woke from an adequate sleep (homeostasis), and by internal circadian rhythms. Thus, the body is ready for sleep and for wakefulness at different times of the day.
The circadian rhythm sleep disorders are:
Among people with healthy circadian clocks, there is a continuum of chronotypes from "larks" or "morning people" who prefer to sleep and wake early, to "owls" who prefer to sleep and wake at late times. Whether they are larks or owls, people with normal circadian systems:
Researchers have placed volunteers in caves or special apartments for several weeks without clocks or other time cues. Without time cues, the volunteers tended to go to bed an hour later and to get up about an hour later each day. These experiments appeared to demonstrate that the "free-running" circadian rhythm in humans was about 25 hours long. However, these volunteers were allowed to control artificial lighting and the light in the evening caused a phase delay. More recent research shows that adults of all ages free-run at an average of 24 hours and 11 minutes. To maintain a 24 hour day/night cycle, the biological clock needs regular environmental time cues, e.g. sunrise, sunset, and daily routine. Time cues keep the normal human circadian clock aligned with the rest of the world.
As of October 1, 2005, the diagnostic codes for circadian rhythm sleep disorders were changed from the 307-group to the 327-group in the Diagnostic and Statistical Manual of Mental Disorders, Fourth Edition, Text Revision (DSM-IV-TR). The DSM updated to agree with the International Classification of Diseases (ICD-9). The new codes reflect the moving of these disorders from the Mental Disorders section to the Neurological section in the ICD.