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.

Ultradian Bipolar Disorder

Ultradian cycling is also related to rapid cycle bipolar disorder, holding the distinction from the normal definition of rapid cycling (having four or more manic or depressive episodes in a year). Ultridian cyclying is going through several depressive and manic episodes in a single day, or in a period that centers around a 24 hour time frame. The scientific connection between bipolar disorder and ultradian cycling has not been proven, but to those who experience it, the characteristic need no validation to be debilitatingly real.

=References= http://polyphasing.blogspot.com/2006/01/quote-on-4-hr-ultradian-cycle.html (Dr. Claudio Stampi is a sleep expert).

Biology

Ultradiane Rhythmik