noun, plural: infradian rhythms
Biological rhythms pertain to the synchronized cyclic pattern demonstrated by an organism as its response to a particular stimulus. Biological clock that synchronizes with biological rhythm may be endogenous or exogenous. An endogenous type is one in which the internal biological clock is the one that controls it. An exogenous type is one that which involves an external cue (i.e. zeitgeber). There are the different types of biological rhythms: circadian rhythms, diurnal rhythms, ultradian rhythms, and infradian rhythms.1
An infradian rhythm has a longer period than a circadian rhythm. The cycle is repeated more than the 24-hour circadian day. This is in contrast to the ultradian rhythm wherein it recurs in periods shorter than a day but longer than an hour.2 In infradian rhythm, the frequency is less than one cycle in 28 hours.3
1 Hedge, A. (2013). Biological Rhythms. Retrieved from []
2 "Ultradian, adj." Oxford English Dictionary. Retrieved from [].
3 Brown, F. M. & Graeber, R. C. (1982). The Rhythmic Aspects of Behavior. Hillsdale, NJ: Lawrence Erlbaum Associates. p.462.