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A chronobiological term that refers to the natural endogenous period of a rhythm when zeitgebers are removed. In humans, it most commonly is seen in the tendency to delay some circadian rhythms, such as the sleep-wake cycle, by approximately 1 hour every day, when a person has an impaired ability to entrain or is without time cues.


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