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cyclic fluctuations in the intensity of various biological processes with an approximate period of 20 to 28 hours. The diurnal rhythms occurring in organisms under natural conditions are frequently regarded as circadian rhythms. It has been shown that in an isolated room, under conditions of constant light, darkness, and temperature the periodicity of biological rhythms in plants and animals (including man) deviates from the 24-hour cycle of diurnal rhythms. If conditions do not change, the periodicity remains stable. In animals active mostly at the end of the day, in the evening, or at night, the period of the circadian rhythms is usually the shortest in darkness; the higher the level of constant illumination, the longer the period. The reverse is true of animals that are more active in the morning and at midday.
According to the most widely held theory, circadian rhythms, regardless of the period, are a strictly spontaneous (endogenous) and genetically fixed cyclicity of the biological processes functioning in the organism. The rhythms become diurnal under the influence of cyclical external conditions. According to another theory, circadian rhythms arise from inherited diurnal rhythms influenced by constant unnatural conditions forced upon the organism. For example, if the constant conditions are favorable, an animal becomes active before the usual time. If they are unfavorable, the time of activity lags every day and the period of the original 24-hour rhythm shortens or lengthens accordingly.
Circadian rhythms can affect the behavior of the integral organism (for example, the depositing of eggs by insects or changes in the position of leaves on a plant) and individual physiological processes. When conditions are constant, circadian rhythms of various functions frequently differ. For example, the periods of rhythms of body temperature and sleep and wakefulness change in humans when light is constant. This lack of coordination of functions in time frequently results in a pathological disorder and is thus of great importance for medicine, especially in connection with space flights of humans and animals. Annual endogenous rhythms apparently lose their stability in a similar fashion and are transformed into biannual rhythms.
REFERENCETsirkadnye ritmy cheloveka i zhivotnykh. Frunze, 1975.
V. B. CHERNYSHEV
Circadian rhythms are biological cycles that take place every twenty-four hours. Human body temperature, for example, rises and falls in regular twenty-four-hour cycles. Among other behaviors, these cycles make it natural to sleep during the night and to be awake in the daytime. When circumstances such as night-shift work dictate otherwise, the natural sleep cycle—and, as a consequence, the regular dream cycle—is altered. Shift workers often sleep less, for example, as a result of the greater state of physiological arousal that accompanies the circadian rhythm’s daylight cycle. Current research indicates that reduction in total sleep time appears to be at the expense of rapid eye movement (REM) sleep, the sleep stage during which our most vivid dreaming occurs. Thus, day sleepers tend to have a less active dream life than night sleepers.