BIORHYTHMS. - By b. are meant the rhythmic activities of living beings. In reality, every vital phenomenon is rhythmic: the very life of a species, with the birth, growth, and death of successive generations of individuals, is rhythmic; a rhythm can be recognized in almost all organic functions: respiration, circulation, secretory and excretory phenomena, etc. Thus many activities of living beings are rhythmic: waking and sleep, feeding, reproduction, etc. The activity of the individual cells composing organisms is also rhythmic.
Biological cycles fall within b., although they are considered separately, since they represent a series of phenomena connected with living beings, more or less complex, by which one returns from a given condition to the same condition. Thus the life cycle from egg to egg, the cycle of organization of the elements composing living matter, such as carbon and nitrogen, the life cycle of parasites, etc. (v. CICLI BIOLOGICI).
The rhythmic character of vital phenomena must be sought in the rhythmic character of celestial phenomena, essentially the rotation of the earth around the sun and around its axis, and the rotation of the moon, and in the phenomena of the physical world in which plants and animals live, such as night and day, the seasons, tides, winds, etc.