An infradian rhythm is a rhythm with a period longer than the period of a circadian rhythm

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...

, i.e. with a frequency less than one cycle in 28 hours, such as menstruation, breeding, tidal or seasonal rhythms. In contrast, ultradian

Ultradian

Ultradian rhythms are recurrent periods or cycles repeated throughout a 24-hour circadian day. In contrast, infradian rhythms, such as the human menstrual cycle, have periods longer than a day....

 refers to rhythms with periods shorter than the period of a circadian rhythm.

Infradian rhythms in menstruation

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....

 is an example of an infradian rhythm, the timing of which is governed by endogenous

Endogenous

Endogenous substances are those that originate from within an organism, tissue, or cell. Endogenous retroviruses are caused by ancient infections of germ cells in humans, mammals and other vertebrates...

 (internal) biological cycles and possibly also by exogenous

Exogenous

Exogenous refers to an action or object coming from outside a system. It is the opposite of endogenous, something generated from within the system....

 cues (zeitgeber

Zeitgeber

Zeitgeber is any exogenous cue that synchronizes an organism's endogenous time-keeping system to the earth's 24-hour light/dark cycle. The strongest zeitgeber, for both plants and animals, is light...

s). One exogenous cue may be female pheromone

Pheromone

A pheromone is a secreted or excreted chemical factor that triggers a social response in members of the same species. Pheromones are chemicals capable of acting outside the body of the secreting individual to impact the behavior of the receiving individual...

 production, which may bring women's menstrual cycles into synchronization with those of other women with whom they are in close proximity for an extended period of time. This is known as the dormitory effect or the McClintock effect

McClintock effect

Menstrual synchrony is a phenomenon reported in 1971 wherein the menstrual cycles of women who lived together reportedly became synchronized over time...

.