Infradian

Various rhythms and cycles in the physiological and pathological contexts can be examined. Specifically, circadian rhythms last about 24 h. Rhythms that are longer than 24 h are called infradian rhythms, such as the menstrual cycle [31, 32], Cycles that are shorter than 24 h are called ultradian rhythms, such as the firing rate of neurons and the rhythm of heartbeats. Currently in biomedicine, the circadian rhythm is the one that has been studied the most extensively [32], More studies on other patterns are still needed. In addition to biological and environmental rhythms, social rhythms and cycles such as school years, as well as the frequencies of biological, environmental, and social events should also be considered. 

Haus and Halberg (19 ) have further categorized rhythms (by time frame) as infradian, circadian, and ultradian. Circadian rhythms are the rhythms that have been studied most extensively and have periods in the range 20-28 hr (therefore, frequencies are about 0.04 cycles per hour). There are many examples that can be cited, including rhythms in mitotic activity, metabolic processes, and susceptibility to drugs. 

Infradian rhythms have periods longer than 28 hr and therefore their frequencies are correspondingly lower than circadian. Some of the well known infradian rhythms are the human menstrual cycle and the annual reproductive cycle of salmon. Infradian rhythms have been identified in nutrient intake and metabolism of foodstuffs (Reinberg, 1983). A more specific type of infradian rh5rthm is the circasemiseptan (period approximately 3.5 day) found by Schweiger et aL (1986). 

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