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Non-24-hour sleep–wake disorder (non-24), is one of several types of chronic circadian rhythm sleep disorders (CRSDs). It is defined as a "complaint of insomnia or excessive sleepiness related to abnormal synchronization between the 24-hour light–dark cycle and the endogenous circadian rhythms of sleep and wake propensity."〔 Symptoms result when the non-entrained (free-running) endogenous circadian rhythm drifts out of alignment with the desired or conventional sleep–wake schedule. However, the sleep pattern can be quite variable; some individuals 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 major social and occupational consequences. People with non-24 "resemble free-running, normal individuals living in a time-isolation facility with no external time cues", while using artificial lighting at will. The majority of patients with non-24 are totally blind, and the failure of entrainment is explained by an absence of photic input to the circadian clock.〔 However, the disorder can also occur in sighted people for reasons that are not well understood. Though often referred to as non-24, for example by the FDA, it is also known by the following terms: * Non-24-hour sleep–wake syndrome〔 * Non-24-hour sleep-wake disorder * Non-24-hour sleep-wake rhythm disorder * Free running disorder (FRD) * Hypernychthemeral disorder * Circadian rhythm sleep disorder – free-running type * Circadian rhythm sleep disorder – nonentrained type * N24HSWD * Non-24-hour circadian rhythm disorder The disorder is an invisible disability that can be "extremely debilitating in that it is incompatible with most social and professional obligations". ==Mechanisms== The internal circadian clock, located in the hypothalamus of the brain, generates a signal that is slightly longer (occasionally shorter) than 24 hours. Normally, this slight deviation is corrected by exposure to environmental time cues, especially the solar 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 more (occasionally less) than 24 hours, expressing the intrinsic period of the 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. The circadian clock modulates many physiological rhythms. The most easily observed of these is the propensity for sleep and wake; thus, patients with non-24 experience symptoms of insomnia and daytime sleepiness (similar to "jet lag") when their endogenous circadian rhythms drift out of synchrony with the social/solar 24-hour day and they attempt to conform to a conventional schedule. Eventually, their circadian rhythms will drift back into normal alignment, and symptoms temporarily resolve, only to recur as their clock drifts out of alignment again. Thus the overall pattern involves recurring symptoms on a weekly or monthly basis, depending on the length of the internal circadian cycle. For example, an individual with a circadian period of 24.5 hours would drift 30 minutes later each day and would be maximally misaligned every 48 days. If patients set their own schedule for sleep and wake, aligned to their endogenous non-24 period (as is the case for most sighted patients with this disorder), symptoms of insomnia and wake-time sleepiness are much reduced. However, such a schedule is incompatible with most occupations and social relationships. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Non-24-hour sleep–wake disorder」の詳細全文を読む スポンサード リンク
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