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delayed sleep phase

A condition that occurs when the clock hour at which sleep normally occurs is moved back in time within a given 24-hour sleep-wake cycle. This results in a temporarily displaced, that is delayed, occurrence of sleep within the 24-hour cycle. The same term denotes a circadian rhythm sleep disturbance, called the delayed sleep phase syndrome.

Definition reprinted with permission from Culebras, Antonio, Clinical Handbook of Sleep Disorders, Butterworth-Heinemann