Non-24-hour sleep–wake disorder
| Non-24-hour sleep–wake disorder | |
|---|---|
| Classification and external resources | |
| Specialty | neurology |
| ICD-10 | G47.24 |
| ICD-9-CM | 327.34 |
| MeSH | D021081 |
Non-24-hour sleep–wake disorder (non-24), is one of several chronic circadian rhythm sleep disorders (CRSDs). It is defined as a "chronic steady pattern comprising [...] daily delays in sleep onset and wake times in an individual living in society." Symptoms result when the non-entrained (free-running) endogenous circadian rhythm drifts out of alignment with the light/dark cycle in nature.
The sleep pattern can be quite variable. People with a circadian rhythm that is quite near to 24 hours may be able to sleep on a conventional, socially acceptable schedule, that is, at night. Others, with a "daily" cycle upwards of 25 hours or more may need to adopt a sleep pattern that is congruent with their free-running circadian clock, shifting their sleep times daily, thereby often obtaining satisfactory sleep but suffering social and occupational consequences.
The majority of people with non-24 are totally blind, and the failure of entrainment is explained by an absence of photic input to the circadian clock. These people's brains may have normal "body clocks", but the clocks do not receive input from the eyes about environmental light levels, as that requires a functioning retina, optic nerve and visual processing center.
The disorder also occurs in sighted people for reasons that are not well understood. Their circadian rhythms are not normal, often running to more than 25 hours. Their visual systems may function normally but their brains are incapable of making the large adjustment to a 24-hour schedule.
Though often referred to as non-24, for example by the FDA, the disorder is also known by the following terms:
The disorder in its extreme form is an invisible disability that can be "extremely debilitating in that it is incompatible with most social and professional obligations".
The internal circadian clock, located in the hypothalamus of the brain, generates a signal that normally is slightly longer (occasionally shorter) than 24 hours, on average 24 hours and 11 minutes. This slight deviation is, in almost everyone, corrected by exposure to environmental time cues, especially the light–dark cycle, which reset the clock and synchronize (entrain) it to the 24-hour day. Morning light exposure resets the clock earlier, and evening exposure resets it later, thereby bracketing the rhythm to an average 24-hour period. If normal people are deprived of external time cues (living in a cave or artificial time-isolated environment with no light), their circadian rhythms will "free-run" with a cycle of a little more (occasionally less) than 24 hours, expressing the intrinsic period of each individual's circadian clock. The circadian rhythms of individuals with non-24 can resemble those of experimental subjects living in a time-isolated environment, even though they are living in normal society.
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