Definition reprinted with permission from Culebras, Antonio, Clinical Handbook of Sleep Disorders, Butterworth-Heinemann
An innate daily fluctuation of physiologic or behavioral functions, including sleep-wake states generally tied to the 24-hour daily dark-light cycle. It sometimes occurs at a measurably different periodicity (e.g., 23 or 25 hours) when light and dark and other time cues are removed.
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