a cyclic repetition (intensification or weakening) of biological phenomena or processes that occurs at daily intervals. There are both solar diurnal rhythms (24 hours) and lunar, or tidal, diurnal rhythms (24.8 hours). The diurnal rhythms that affect the earth’s inhabitants are one of the most distinctive biological rhythms. They arise from the organism’s reactions to diurnal changes in the environment and from spontaneous physiological rhythms. Under the influence of unchanging conditions, spontaneous diurnal rhythms may become circadian rhythms. Spontaneous diurnal rhythms also occur in individual cells, including cells artificially deprived of their nucleus.

Man has several dozen physiological functions that exhibit diurnal rhythms. The diurnal changes in the sensitivity of animals and plants to such chemical and physical influences as those of medicine, poisons, and radiation are of practical importance. Disturbance of the natural rhythm of external conditions desyn-chronizes the diurnal rhythms of various physiological functions and eventually results in disease. Similar phenomena occur when animals and plants are raised (grown) under artificial conditions and when humans are isolated from their natural environment, as in space flight.