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Non-24-hour sleep–wake rhythm disorder (N24HSWRD) is a circadian rhythm disorder in which an individual’s endogenous period (tau period) is not entrained to the 24-hour day. 1 This causes a day-by-day forward shift in sleep–wake timing. Each cycle, lasting a few weeks, comprises periods of normal sleep (when the intrinsic sleep time occurs at night) and periods of insomnia and daytime hypersomnolence (when the endogenous sleep time occurs during the day).