Ultradian rhythm

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.

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

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.