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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. In contrast, ultradian refers to rhythms with periods shorter than the period of a circadian rhythm.
- Brown, Frederick M. & Graeber, R. Curtis, eds. The Rhythmic Aspects of Behavior. Hillsdale, NJ: Lawrence Erlbaum Associates. 1982, (p.462).
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