Ultradian rhythms are recurrent periods or cycles repeated throughout a 24-hour circadian day.
The term ultradian is used in sleep research in reference to the 90–120 minute cycling of the sleep stages during human sleep. (84)
Some of the other ultradian cycles of the body are hormonal release, heart rate, thermoregulation, urination, bowel activity, nostril dilation and appetite. The last involves rhythmic release of Neuropeptide Y (NPY) and Corticotrophin-releasing hormone (CRH), stimulating and inhibiting appetite ultradian rhythms.
Ultradian mood 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.
Other rhythms associated with thermoregulation
Circamensal rhythm. Body temperature is sensitive to many hormones, so women have a temperature rhythm that varies with the menstrual cycle, called a circamensal rhythm. Hormonal contraceptives both suppress the circamensal rhythm and raise the typical body temperature by about 0.6 °C, or 1.1 °F.
Circannual rhythm. Temperature also varies with the change of seasons during each year. This pattern is called a circannual rhythm. Circannual rhythm involves physiological process. It fluctuates at interval of 1 year even in control condition of temperature, light and other seasonal clues have been eliminated. Studies of seasonal variations have produced inconsistent results. People living in different climates may have different seasonal patterns.