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.

Associated with hormonal release, heartbeat, appetite and thermoregulation. Rhythmic release of Neuropeptide Y (NPY) and Corticotropin-releasing hormone (CRH) involved in appetite stimulation and inhibition, respectively, are examples of ultradian rhythms.