Definitions of Rhythms

A rhythm is characterized as a repeatable change in a measurement that occurs over time. Rhythms are classified based on the frequency of the rhythm.1-3 Those rhythms that occur less than once per day are ultradian rhythms and include the electroen-cephalographic (EEG) rhythm, heart rate, and hormonal pulses (Figure 12.1). Those rhythms that occur on a daily basis and are endogenously generated are circadian rhythms. Conversely, those rhythms that occur on a daily basis but are maintained by other factors (such as the light/dark cycle) are diurnal rhythms. Examples of circadian or diurnal rhythms include the body temperature rhythm and the sleep/wake cycle. Rhythms that occur approximately once a month are circalunar rhythms and those that occur on a yearly basis are circannual rhythms.

FIGURE 12.1

A time scale depiction of the nomenclature used when describing rhythmic processes. All rhythms shorter than 24 h are classified as ultradian rhythms and include heart rate, hormone pulses, and electroenceph-alographic (EEG) recordings. Rhythms with a period of approximately 24 h can be diurnal or circadian. Diurnal rhythms are controlled by another factor (such as the light:dark cycle) and will not be observed if the controlling factor is removed. Circadian rhythms are endogenously generated and will continue even in constant conditions with external sources merely acting as entrainment cues. Examples of circadian rhythms include the body temperature rhythm and general activity. Rhythms that occur on a monthly basis are termed circalunar, while those that occur on a yearly basis are circannual.

FIGURE 12.1

A time scale depiction of the nomenclature used when describing rhythmic processes. All rhythms shorter than 24 h are classified as ultradian rhythms and include heart rate, hormone pulses, and electroenceph-alographic (EEG) recordings. Rhythms with a period of approximately 24 h can be diurnal or circadian. Diurnal rhythms are controlled by another factor (such as the light:dark cycle) and will not be observed if the controlling factor is removed. Circadian rhythms are endogenously generated and will continue even in constant conditions with external sources merely acting as entrainment cues. Examples of circadian rhythms include the body temperature rhythm and general activity. Rhythms that occur on a monthly basis are termed circalunar, while those that occur on a yearly basis are circannual.

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