BIOLOGICAL CLOCK: ULTRADIAN RHYTHM
The study of biological clocks and rhythms has now evolved into a special science called chronobiology. Chronobiologists are not only interested in the biological rhythm of sleep but also in other biological functions such as the rhythm of hormonal secretion, urinary excretion, gastric function, body temperature, and the periodical fluctuation of human performance during the 24 hour cycle. Some chronobiologists are even interested in the biological rhythms in animals and plants.
Franz Halberg, a US chronobiologist, divided biological rhythms into three types. In 1959 he described the familiar circadian rhythm— its period is about a day. Then, in 1967, he described ultradian rhythms, which are biological rhythms with periods of less than a day. These include the 90 minute sleep cycle, also known as the REM/NREM cycle or Kleitman’s basic rest activity cycle (BRAC). This has been described in chapter 5, Two kinds of Sleep. Rhythms of the third type are called infradian rhythms. These rhythms have periods in excess of a day; the most familiar is the monthly menstrual period. These biological rhythms can be summarised as follows:
(1) Ultradian rhythms: less than a day
(2) Circadian rhythms: about a day
(3) Infradian rhythms: more than a day
The ultradian rhythm with its periodicity of about 90 minutes determines human performance and arousal state both day and night throughout the 24 hour period. This 90 minute cycle has also been found to operate in other human biological activities; for example, urinary volume and concentration, gastric contraction activity, pupil size under constant illumination, respiratory rate, and even heart rate. This rhythm exerts its influence both in the awake and in the sleeping state.
The periodicity of the ultradian rhythm varies from species to species. In man, it is about 90 minutes; in cats, 20 minutes; in rhesus monkeys, 60 minutes. The significance of this ultradian rhythm and the reason for its existence is still a mystery.
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