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. 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 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. 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.

Normal circadian rhythms

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:

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.
After starting a new routine which requires they get up earlier than usual, start to fall asleep at night earlier within a few days. For example, someone who is used to sleeping at 1 am and waking up at 9 a.m. begins a new job on a Monday, and must get up at 6 a.m. to get ready for work. By the following Friday, the person has begun to fall asleep at around 10 p.m., and can wake up at 6 a.m. feeling 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 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.