Ultradian rhythms are recurrent periods or cycles repeated throughout a 24-hour circadian
Circadian rhythm
A circadian rhythm, popularly referred to as body clock, is an endogenously driven , roughly 24-hour cycle in biochemical, physiological, or behavioural processes. Circadian rhythms have been widely observed in plants, animals, fungi and cyanobacteria...

 day. In contrast, infradian rhythm
Infradian rhythm
An infradian rhythm is a rhythm with a period longer than the period of a circadian rhythm, i.e. with a frequency less than one cycle in 28 hours, such as menstruation, breeding, tidal or seasonal rhythms...

s, such as the human menstrual cycle
Menstrual cycle
The menstrual cycle is the scientific term for the physiological changes that can occur in fertile women for the purpose of sexual reproduction. This article focuses on the human menstrual cycle....

, have periods longer than a day.

The descriptive term ultradian is used in sleep
Sleep is a naturally recurring state characterized by reduced or absent consciousness, relatively suspended sensory activity, and inactivity of nearly all voluntary muscles. It is distinguished from quiet wakefulness by a decreased ability to react to stimuli, and is more easily reversible than...

 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 hormonal release, heart rate
Heart rate
Heart rate is the number of heartbeats per unit of time, typically expressed as beats per minute . Heart rate can vary as the body's need to absorb oxygen and excrete carbon dioxide changes, such as during exercise or sleep....

, thermoregulation
Thermoregulation is the ability of an organism to keep its body temperature within certain boundaries, even when the surrounding temperature is very different...

, urination
Urination, also known as micturition, voiding, peeing, weeing, pissing, and more rarely, emiction, is the ejection of urine from the urinary bladder through the urethra to the outside of the body. In healthy humans the process of urination is under voluntary control...

, bowel activity, nostril dilation
Nasal cycle
The nasal cycle is the alternating congestion and decongestion of the nasal cavities in humans. It is a physiological congestion of the nasal concha due to selective activation of one half of the autonomic nervous system by the hypothalamus. It should not be confused with pathological nasal...

 and appetite
The appetite is the desire to eat food, felt as hunger. Appetite exists in all higher life-forms, and serves to regulate adequate energy intake to maintain metabolic needs. It is regulated by a close interplay between the digestive tract, adipose tissue and the brain. Decreased desire to eat is...

. The last involves rhythmic release of Neuropeptide Y
Neuropeptide Y
Neuropeptide Y is a 36-amino acid peptide neurotransmitter found in the brain and autonomic nervous system."NPY has been associated with a number of physiologic processes in the brain, including the regulation of energy balance, memory and learning, and epilepsy." The main effect is increased food...

 (NPY) and Corticotropin-releasing hormone
Corticotropin-releasing hormone
Corticotropin-releasing hormone , originally named corticotropin-releasing factor , and also called corticoliberin, is a polypeptide hormone and neurotransmitter involved in the stress response...

 (CRH), stimulating and inhibiting appetite ultradian rhythms.

Caenorhabditis elegans
Caenorhabditis elegans
Caenorhabditis elegans is a free-living, transparent nematode , about 1 mm in length, which lives in temperate soil environments. Research into the molecular and developmental biology of C. elegans was begun in 1974 by Sydney Brenner and it has since been used extensively as a model...

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
Bipolar disorder
Bipolar disorder or bipolar affective disorder, historically known as manic–depressive disorder, is a psychiatric diagnosis that describes a category of mood disorders defined by the presence of one or more episodes of abnormally elevated energy levels, cognition, and mood with or without one or...

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.
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