Ultradian rhythms are recurrent periods or cycles repeated throughout a 24-hour day. (In contrast, infradian rhythms have periods longer than a day, for example the human menstrual cycle.)

The descriptive term ultradian is 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 a 90 minute REM cycle in human sleep.

Some of the ultradian cyclings of the body are hormonal release, heart rate, thermoregulation, nostril dilation and appetite. The last involves rhythmic release of Neuropeptide Y (NPY) and Corticotropin-releasing hormone (CRH), stimulating and inhibiting appetite ultradian rhythms.

Ultradian states in bipolar disorder cycle much faster than rapid cycling, which is defined as four or more mood episodes in one year, sometimes occurring within a few weeks. Ultradian mood cycling is characterized by cycles shorter than 24 hours.

References

• Ultra-rapid and ultradian cycling in bipolar affective illness. Kramlinger KG, Post RM. Br J Psychiatry. 1996 Mar;168(3):314-23. Associated Articles and Links [1]