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Ultradians are the regular recurrence in cycles of less than 24 hours from one stated point to another, as certain biologic activities which occur at such intervals, regardless of conditions of illumination.

Commonly used in sleep research to describe individual stages of sleep that occur within intervals of an organism's circadian rhythm, and especially to refer to the 4-hour ultradian cycle.

Hormonal release, heartbeat, appetite, nostril dilation and thermoregulation are ultradial cycles. Rhythmic release of Neuropeptide Y (NPY) and Corticotropin-releasing hormone (CRH) involved in appetite stimulation and inhibition, respectively, are examples of ultradian rhythms.

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Quote on the 4-hr Ultradian Cycle (Dr. Claudio Stampi is a sleep expert).

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