Circadian Sleep Disorders Network - Advocating for people with misaligned body clocks
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Brief Definitions
Circadian Rhythm
Circadian Sleep Disorders
Jet Lag
Shift Work

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What Are Circadian Rhythm Sleep Disorders?

Some brief definitions. More detailed descriptions appear below (click on the links).
Circadian Rhythms
Physiological and behavioral changes in the body that occur on roughly a 24 hour cycle, sometimes called the body clock
Circadian Rhythm Sleep Disorders (CRSDs)

(also called
Circadian Rhythm Disorders (CRDs)
Circadian Sleep Disorders (CSDs)
Abnormalities in length, timing, and/or rigidity of the sleep-wake cycle relative to the day-night cycle

Five types of CRDs are defined below. The first four are chronic, with neurological causes. The last is temporary, with social and environmental causes

Delayed Sleep Phase Syndrome (DSPS)
Delayed Sleep Phase Disorder (DSPD)
A condition characterized by an inability to fall asleep until very late at night, with the resulting need to sleep late in the morning or into the afternoon; but an ability to sleep reasonably well if sleep and wake times are much later than normal.
Non-24-Hour Sleep-Wake Disorder (Non-24)
A condition in which a person's day length is longer than 24 hours. Sleep times get progressively later and later, cycling completely around the clock.
Advanced Sleep Phase Syndrome (or Disorder) (ASPS or ASPD)
A condition characterized by a need to sleep and wake up much earlier than normal
Irregular Sleep-Wake Disorder (ISWD)
A condition characterized by irregular sleep and wake periods, at least three sleep periods per day
Shift Work Disorder
A condition in which circadian rhythms are disturbed due to working during the body's natural sleep time, and the patient has serious difficulty in adjusting to the required schedule.

And a couple of other definitions:

Jet Lag
A condition in which different body cycles are temporarily out of sync with each other and with the day-night cycle, resulting from travel across time zones. This is a temporary condition and is no longer classified as a circadian rhythm sleep disorder.
Reverse Sleep
This term has been used by some to refer to people who sleep during daylight and are awake at night. It could refer to either DSPD or Non-24.

Circadian Rhythm

Circadian means "roughly daily". The word was coined some 50 years ago from the Latin terms circa, about, and diem, day. Circadian rhythms cycle daily according to the 24-hour rotation of the earth, and they are internally produced in all living things.

Circadian rhythms allow organisms to anticipate what will happen soon and adjust physical and behavioral changes accordingly. They also allow seasonal animals and plants to keep track of the seasons by measuring daylength.

Circadian rhythms are often called the body clock. Humans produce on average a cycle lasting a bit over 24 hours, though there are individual variations. While these internal rhythms are approximately 24 hours, they are adjusted daily by external factors, especially sunlight or other bright lights.

The most noticeable feature of circadian rhythms is the sleep/wake cycle. But there are other circadian rhythms including swings in many hormones throughout the day and night, the body temperature cycle, appetite and the best times of alertness and productivity. Ideally these rhythms are in sync with each other and with the light-dark cycle in nature, the norm being wakefulness during daylight hours and sleep during darkness. So for example normal adults usually go to sleep between 10 PM and 1 AM and awaken 7-8 hours later with no problem.

Some people are flexible and can adjust to sleeping on practically any shift. Still, they may prefer to wake up early (such people are often called "morning larks") or stay up late ("night owls"). Other people cannot adjust, and sleeping at the wrong time can make them ill. These people have circadian sleep disorders.

Circadian Rhythm Sleep Disorders

Chronic circadian rhythm sleep disorders are internal (endogenous, or built-in) abnormalities of the circadian rhythms, the body's internal clock. They arise when a person is unable to fall asleep at a normal bedtime (late evening), although he/she is able to sleep at other times.

The length of the internal circadian cycle can normally be a bit shorter or longer than 24 hours. The cycle is entrained to 24 hours by external factors, especially light. If it cannot be entrained, either because it is too far afield of the normal range, or for other neurological reasons, the result is a circadian disorder. In these disorders, the internal coordination of the various rhythms may also be normal or faulty. For example, some hormones may be on a different cycle than others.

Common to these disorders is inflexibility: even when physically tired or sleep deprived, sufferers cannot make up for lost sleep outside of their hard-wired sleep times. This factor is generally misunderstood by people who do not suffer from these disorders, leading to misunderstanding of what sufferers are up against, and a conclusion that they are just lazy or haven't tried hard enough to live on a normal schedule.

The International Classification of Diseases (ICD-10-CM, 2014) lists 6 subtypes of circadian rhythm sleep disorder:

as well as unspecified, other, and conditions specified elsewhere.

The DSM-V defines Circadian Rhythm Sleep-Wake Disorder as follows:

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 individual's physical environment or social or professional schedule.

The sleep disruption leads to excessive sleepiness or insomnia, or both.

The sleep disturbance causes clinically significant distress or impairment in social, occupational, and other important areas of functioning.

Delayed Sleep Phase Disorder (DSPD)

Delayed Sleep-Phase Disorder (DSPD), also called Delayed Sleep Phase Syndrome (DSPS), is a circadian sleep disorder in which the individual's internal body clock is delayed with respect to the external day/night cycle.

A person with DSPD naturally falls asleep late at night, typically between 1:00 am and 6:00 am, and awakens in the late morning or in the afternoon. There is a striking inability to fall asleep at an earlier, more typical bedtime. As a result, many people with DSPD have been labeled as insomniacs. But if such a person is allowed to follow his internal body clock, he generally has no problems with either falling asleep or waking naturally.

However, if he ignores his internal clock and attempts to live on a normal schedule, he finds himself unable to fall asleep until very late, but is forced to wake up early nonetheless. Despite being tired all day, he finds himself unable to fall asleep any earlier the next night. DSPD sufferers therefore differ from typical night owls who prefer to stay up late at night but can sleep at earlier times when they desire. A person with DSPD has no choice as to what time sleep onset comes.

Thus DSPD has two essential components: one is the body's daily cycle, which is later than normal; the other is the difficulty or even inability to shift one's schedule to an earlier time.

While some individuals with DSPD are able to advance their sleep phase as much as 2 hours earlier, for example from a 5am sleep onset to a 3am sleep onset, by undertaking light therapy and melatonin treatment, they may find it impossible to advance their sleep phase further, to a societally normal day schedule. And many do not have success even with phase advancement of as little as 2 hours.

For people with severe DSPD, a sleep onset change for any extended period of time has proven to be impossible, and therefore such people must sleep the hours set by their neurological sleep mechanism. The only true solution for such people, to avoid the destructive physical and mental deterioration that so many do experience, is for them to stay on the sleep schedule that is their natural physiologically dictated sleep schedule. Of course, that results in difficulty meeting the demands of education, employment, relationships, and parenting.

The lack of sleep resulting from trying to fit into a normal wake time can cause difficulty thinking clearly, driving safely, and generally functioning well. Over time, this sleep deprivation may significantly reduce a person's productivity and enjoyment of life, and can lead to frequent illness, clinical depression, or other stress-related medical problems.

There seem to be two types of DSPD. The adolescent variety, most common among boys, is outgrown by the early 20s. The literature says that this temporary form of DSPD may affect as many as 10% of teenagers. Lifelong DSPD can begin in early childhood or puberty. Studies in the 1990s showed that about 0.15% of adults are affected, as many women as men.

DSPD was first formally described in 1981 by Dr. Elliot D. Weitzman and others at Montefiore Medical Center. It is thought to be responsible for 7-10% of patient complaints of chronic insomnia. However, as many doctors are still unfamiliar with it, it often goes untreated or is treated inappropriately. DSPD is often misdiagnosed as primary insomnia, ADHD, depression, or other psychiatric conditions.

Questions? Read our DSPD Q&A document

Non-24-Hour Sleep-Wake Disorder (Non-24)

Non-24-hour Sleep-Wake Disorder (Non-24), also known as hypernychthemeral syndrome or circadian rhythm sleep disorder, free-running type, is a circadian sleep disorder in which an individual would naturally fall asleep later and later each day, eventually rotating all they way around the clock. Generally the delay is about an hour or two, corresponding to a circadian cycle of 25 - 26 hours; but some individuals with Non-24 exhibit a much longer delay, especially those who have lived with Non-24 for many years. Non-24 is most commonly diagnosed in people who are totally blind, but a significant number of sufferers are sighted.

In blind people with no light perception Non-24 is presumably due to the lack of light cues that would keep a person's circadian rhythm synchronized to the external light/dark cycle. In sighted individuals it is believed to have the same causes and mechanisms as DSPD, but more extreme.

The daily delay in sleep onset causes the individual to gradually "cycle around the clock." This means that part of the month the individual will be asleep at night and awake during the day, but another part will be lived in complete or near complete darkness, which can be disconcerting and depressing for the person with Non-24.

This disorder is extremely debilitating. The lack of a stable sleep time makes it very difficult, often impossible, for a person with Non-24 to maintain those things in life that occur at regular times -- employment, appointments with doctors, marriage and family life, and other social interactions. Thus Non-24 can lead to isolation, poor health, poverty, and depression.

An individual struggling to maintain life reponsibilities despite Non-24 may have limited success for a time, but Non-24 tends to become more rigid over time. The chronic sleep deprivation that comes with fighting against the body's urges to sleep and the increasing difficulty in fighting those urges as the disorder becomes more rigid (more entrenched, more severe) will eventually force those who do not respond to treatment into admitting that they have become disabled, giving up their efforts to remain in the workplace, and seeking alternate means of meeting their life's needs.

It is unknown exactly how many people with Non-24 can maintain a partially or totally normal sleep schedule with treatment (light therapy, medications, melatonin, etc.), but one study found that nearly a third of patients responded at least partially to treatment. The study did not indicate how many of those patients experienced complete remission of Non-24 with treatment, but it is assumed that total remission is relatively rare, given the tiny number of people with Non-24 in support communities who report full recovery with treatment.

Questions? Read our Non-24 Q&A document

Advanced Sleep Phase Disorder (ASPD)

Advanced Sleep Phase Disorder (ASPD) (also called Advanced Sleep Phase Syndrome, ASPS) is characterized by bedtime and wake-up time much earlier than normal, although sleep quality is normal. People with ASPD may fall sleep at 6 or 8 p.m. and awaken about eight hours later. While the disorder appears to be much less common than its opposite, DSPS, it may simply be that it is diagnosed less often, since people with ASPD are generally able to maintain a normal 9 - 5 workday without difficulty.

Irregular Sleep-Wake Disorder (ISWD)

Irregular Sleep-Wake Disorder (ISWD) is characterized by at least three sleep episodes per 24-hour period, irregularly from day to day. It most commonly occurs in elderly persons with dementia. It also occurs in some children with developmental disorders, including autism spectrum disorders. Finally ISWD can be a consequence of brain tumors or traumatic brain injury in both children and adults. It can cause social, familial and work problems.

Shift Work Disorder

Shift Work Disorder may occur when work schedules force people to be awake when their circadian rhythms dictate that they should be sleeping. It is classified as a Circadian Rhythm Disorder (CRD) and is extrinsic, i.e. caused by external behavioral factors. A considerable amount of research has been done on shift work disorder because of the importance of shift work in certain industries and occupations.

Rotating and night shifts cause greater problems for some people than for others. Shift work results in a circadian sleep disorder in those people in whom the body clocks cannot shift sufficiently, and the person suffers chronic sleep deprivation.

Jet Lag

Jet Lag results from travelling across time zones. Nighttime begins several hours earlier (or later) in the new time zone, than it did in the old time zone. As a result, the sleep/wake cycle must shift, and all the other circadian rhythms shift also. But they don't all shift together. The shift in sleep hours may in itself cause significant tiredness. The desynchronization of the various circadian processes causes additional fatigue and malaise. Over a period of days, the various body clocks entrain to the new day/night cycle.

A recent article1 presents an intuitive way of looking at jet lag:

One way to think about our circadian rhythm is to imagine a wall of pendulum clocks with rubber bands connecting the pendulums, keeping them in unison. When we cross time zones, our central clock (in a part of the brain known as the suprachiasmatic nucleus, or SCN) receives information about ambient light from the retina, indicating the change in time of day.

This alters the rhythm of the SCN, but because the body's other clocks are only loosely coupled to the central clock, it takes time for them and the organ systems to which they belong to resynchronize—that is, to swing in harmony like the connected clock pendulums. During this time of adjustment, people often experience a mental and physical malaise.

[1] Block, Gene D, Our Internal Sleep Clocks Are Out of Sync (The Wall Street Journal, Aug 14, 2014)
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Last modified Apr 19, 2021
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