Ultradian rhythm

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An ultradian rhythm is a recurrent period or cycle repeated throughout a 24-hour circadian day. In contrast, infradian rhythms, such as the human menstrual cycle, have periods longer than a day. The Oxford English Dictionary's definition of Ultradian specifies that it refers to cycles with a period shorter than a day but longer than an hour.^[1]

The descriptive term ultradian is used in sleep research in reference to the 90–120 minute cycling of the sleep stages during human sleep.^[2]

Some of the other ultradian cyclings of the body are blood circulation, blinking, pulse, hormonal release, heart rate, thermoregulation, urination, bowel activity, nostril dilation and appetite. The last involves rhythmic release of Neuropeptide Y (NPY) and Corticotropin-releasing hormone (CRH), stimulating and inhibiting appetite ultradian rhythms. There is a circasemidian rhythm in body temperature and cognitive function. However, this appears to be the first harmonic of the circadian rhythm of each and not an intrinsic rhythm.

Caenorhabditis elegans is often used as a model animal for ultradian rhythm. Defecation in C. elegans is a tightly controlled rhythmic process. Posterior body wall muscle contractions in C. elegans occur rhythmically every 45–50 seconds and mediate defecation.

Ultradian mood states in bipolar disorder cycle much faster than rapid cycling; the latter is defined as four or more mood episodes in one year, sometimes occurring within a few weeks.^a Ultradian mood cycling is characterized by cycles shorter than 24 hours.^[3]

Notes

• ^a The term rapid cycling was defined prior to discovery of ultradian mood states. Thus it can be misleading to think of these states as ultra-rapid cycling, as this particular form of cycling is also described as a mixed state, and has not gained universal acceptance. Both terms are unable to capture the nature of this unique form of manic-depressive illness.^[3]

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