The rhythm of daily activities and bodily processes such as eating, defecating or sleeping, frequently controlled by hormones, which repeats every twenty-four hours.
An internal system in organisms that influences behavior in a rhythmic manner. Functions such as growth, feeding, secretion of hormones, the rate of drug action, the wake-sleep cycle, the menstrual cycle, and reproduction coincide with certain external events such as day and night, the tides, and the seasons. Biological clocks appear to be set by environmental conditions in some animals, but if these animals are isolated from their environment they continue to function according to the usual rhythm. A gradual change in environment does produce a gradual change in the timing of the biological clock.
A comparable term for the body’s circadian rhythm, the intrinsic biological fluctuations experienced throughout a 24-hour period, is the “diurnal cycle.”
The term “biological clock” is commonly used to describe the intrinsic timing mechanism that is believed to govern physiological processes and cycles in living organisms.