Circadian rhythm sleep disorder
Synonyms and keywords: Biological clock; daytime sleepiness; circadian rhythm sleep-wake disorders; shift worker;  night worker; ultradian rhythm

# Overview
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, and by internal circadian rhythms. Thus, 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, which affects people who travel across several time zones
- Shift work sleep disorder, which affects people who work at night and 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, which causes the affected individual to stay up later and later each night, then wake up later each morning
- Irregular sleep-wake pattern, which 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 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.

# Circadian rhythm abnormalities
Persistent circadian rhythm sleep disorders such as Non-24 hour sleep-wake syndrome are believed to be caused by a reduced ability to reset the sleep/wake cycle in response to environmental time cues. For example, these individuals' circadian clocks might have an unusually long cycle, or might not be sensitive enough to time cues.  People with DSPS, which is more common, do entrain to nature's 24 hours, but are unable to sleep and awaken at socially acceptable times, sleeping instead, for example, from 4 a.m. to noon.

# Differential Diagnosis
Circadian rhythm sleep disorder must be differentiated from:
- Lung disease
- Neuromuscular disorder
- Skeletal malformation
- Other causes of sleep-related hypoxemia

# Epidemiology and Demographics

## Prevalence
The prevalence of circadian rhythm sleep-wake disorders is 1,700 per 100,000 (0.17%) of the general population.

# Risk Factors
Risk factors for circadian rhythm sleep disorders include:
- Longer than average circadian period
- Changes in light sensitivity
- Impaired homeostatic sleep drive
- Mutations in circadian genes (e.g., PER3, CK1e)

# Diagnostic Criteria

## DSM-V Diagnostic Criteria for Circadian Rhythm Sleep-Wake Disorders
- A. A persistent or recurrent pattern of sleep disruption that is primarily due to an alteration of the circadian system or to a misalignment between the endogenous circadian rhythm and the sleep-wake schedule required by an individuals physical environment or social or professional schedule.
AND
- B. The sleep disruption leads to excessive sleepiness or insomnia, or both.
AND
- C. The sleep disturbance causes clinically significant distressor impairment in social, occupational, and other important areas of functioning.