Circadian physiologists think of biological rhythms as either circadian rhythms or everything else. In terms of frequency of oscillation, all rhythms below circadian rhythms are called infradian rhythms, while all rhythms above circadian rhythms are called ultradian rhythms (Figure 4.10). Although no strictly defined boundaries exist, the designation circadian usually is reserved for biological oscillations between 10 and 14 ^Hz (that is, between 10 X 10-6 and 14 X 10-6 cycles per second). Most biological rhythms, however, are described not in terms of frequency of oscillation but in terms of its reciprocal, period. The designation circadian is usually reserved for biological rhythms with periods between 19 and 28 hours. Note, however, that the prefixes infra and ultra are counterintuitive when period is used: infradian rhythms have periods longer than circadian rhythms, while ultradian rhythms have periods shorter than circadian rhythms.
This section of the chapter discusses ultradian rhythms (that is, high-frequency biological oscillations). Infradian rhythms are discussed in Sections 4.3 and 4.4, and circadian rhythms are covered in Chapter 5. Although ultradian and infradian rhythms are not, by definition, circadian rhythms (and, therefore, are not strictly in the
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