The internal circadian clock, located in the hypothalamus of the brain, generates a signal that normally is slightly longer (occasionally shorter) than<unk> <unk> s, on average<unk> <unk> s 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<unk> -hour day. Morning light exposure resets the clock earlier, and evening exposure resets it later, thereby bracketing the rhythm to an average<unk> -hour period. If people who do not have<unk>  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<unk> <unk> s, expressing the intrinsic period of each individual's circadian clock. The circadian rhythms of individuals with<unk> -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, people with<unk> -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<unk> -hour day, but they conform to a conventional schedule. Eventually, their circadian rhythms will drift back into normal alignment, when symptoms temporarily resolve. Thus the overall pattern involves periodic 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<unk> .5<unk> s 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<unk> -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. The AASM suggests that<unk> -24 might be different disorders in sighted and blind people, with different internal and external contributing factors—as these can affect treatment response, different treatments could be needed. Future studies should therefore try to identify and assess these factors.
Non-24-hour sleep–wake disorder
Circadian rhythm sleep disorder
Delayed sleep phase disorder
Irregular sleep–wake rhythm
Advanced sleep phase disorder
Shift work sleep disorder
Excessive daytime sleepiness
Sleep state misperception