Circadian rhythm sleep disorders present with symptoms of insomnia and excessive sleepiness due to an alteration in the circadian timing system resulting in desynchronization between the timing of endogenous circadian rhythms with the 24. h external environment. Jet lag and shift work sleep disorder occur as a response to changes in entrainment cues or perturbations of normal sleep and wake times, whereas advanced sleep phase (ASPS) and delayed sleep phase (DSPS) are thought to be primarily due to fundamental changes in the regulation of circadian timing. ASPS and DSPS are characterized by persistent advances or delays of the sleep and wake period. Genetic study of familial and nonfamilial ASPS and DSPS has demonstrated heritability of these disorders and identified variants in known circadian and noncircadian genes causing these circadian rhythm sleep disorders.
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