Circadian rhythm sleep disorders are a family of sleep disorders affecting 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. In general, they are able to get enough sleep if allowed to sleep when they want. Unless they have another sleep disorder, their sleep is of normal quality.
Humans have biological rhythms, known as circadian rhythms, which are controlled by a biological clock and work on a daily time scale. Due to the circadian clock, sleepiness does not continuously increase as time passes. Instead, the drive for sleep follows a cycle, and the body is ready for sleep and for wakefulness at different times of the day.
Types of circadian rhythm sleep disorders
The circadian rhythm sleep disorders are:
- Jet lag, affecting people who travel across time zones.
- Shift work: People who work at night often have trouble sleeping during the day.
- Delayed sleep phase syndrome (DSPS), which causes difficulty falling asleep at night and waking up in the morning.
- Advanced sleep phase syndrome (ASPS), which causes difficulty staying awake in the evening and staying asleep in the morning.
- Non-24-hour sleep-wake syndrome causes patients to stay up later and later every night, then wake up later every morning.
- 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).
Normal circadian rhythms
Among people with healthy circadian clocks, there are "larks" or "morning people" who prefer to sleep and wake early, and there are "owls" who prefer to sleep and wake at late times. Whether they are larks or owls, people with normal circadian systems:
- can wake in time for what they need to do in the morning, and fall asleep at night in time to get enough sleep before having to get up.
- can sleep and wake up at the same time every day, if they want to.
- will, after starting a new routine which requires they get up earlier than usual, start to fall asleep at night earlier too within a few days. For example, someone who is used to sleeping at 1 am and waking up at 9 am begins a new job on a Monday, and must get up at 6 am to get ready for work. By the following Friday, the person has begun to fall asleep at around 10 pm, and can wake up at 6 am and feel well-rested. This adaptation to earlier sleep/wake times is known as "advancing the sleep phase." Healthy people can advance their sleep phase by about one hour each day.
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 demonstrate that the "free-running" circadian rhythm in humans is about 25 hours long. 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 human circadian clock aligned with the rest of the world.
Circadian rhythm abnormalities
Persistent circadian rhythm sleep disorders such as DSPS are believed to be caused by a reduced ability to reset the sleep/wake cycle in response to environmental time cues. For example, the circadian clocks of individuals with DSPS might have an unusually long cycle, or might not be sensitive enough to time cues.
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