Biological Rhythms

cyclic variations in the intensity and character of biological processes and phenomena. Biological rhythms are observed in almost all animals and plants, unicellular as well as multicellular, in a few isolated organs, and in some cells. Some biological rhythms (heartbeat, respiration frequency, and so on) are relatively independent; others—strictly biological ones—make it possible for organisms to adapt to cyclic changes in the environment (daily, seasonal, and so on).

The solar-diurnal (24 hrs.) rhythm is inherent in most physiological processes (frequency of cell division, variations in body temperature, intensity of metabolism and energy in animals and humans, and so on). It is manifested in the condition and behavior of living organisms by changes in the motor activity of animals, by changes in the positions of leaves and flower petals in plants, by changes in glycogen consumption in the liver of mammals, and other biochemical processes. Neurohumoral centers, which coordinate the 24-hour periodicity of physiological processes, have been discovered in animals. Monophasic and polyphasic 24-hour rhythms are distinguished, depending on the number of activity periods within 24 hours. In the course of individual development (ontogeny) in many animals and in man, a conversion from polyphasic to monophasic rhythm occurs (thus, for infants, repeated alternations of wakefulness and sleep within 24 hours is characteristic).

The lunar-diurnal (24.8 hrs.), or tidal, rhythm is characteristic for the majority of plants and animals in coastal zones and is manifested, along with solar-diurnal rhythm, in variations of motor activity, periodicity of valve opening in mollusks, vertical distribution of small sea animals in a stratum of water, and so on. Solar-diurnal and lunar-diurnal rhythms, as well as astral-diurnal ones (23.9 hrs.), have great significance in the navigation of animals (for example, migratory birds and many insects), which “use” astronomic indicators to orient themselves.

The lunar-monthly rhythm (29.4 days) corresponds to periodicity of changes in the level of ocean tides and is manifested in the rhythmicality of the hatching from cocoons of insects that breed in the coastal zone and in the reproductive cycles of the palolo worm, of certain algae, and of many other animals and plants. Women’s menstrual cycles are also close to the lunar-monthly rhythm.

The annual (seasonal) rhythm of change in the number and activity of animals and of the growth and development of plants is widely known. Annual rhythms in animals and plants are in many cases regulated by the amount of daylight, temperature, and other climatic factors.

Biological rhythms are not only direct reactions to changes in external conditions. They are maintained under artificial conditions—such as constant illumination, temperature, humidity, and atmospheric pressure; moreover, the duration of each period of biological rhythm is practically independent of the intensity of metabolic processes. For instance, chemical substances that slow metabolic processes exert no influence on the diurnal rhythm of spore formation in certain algae; the mass hatching of Drosophila flies occurs every 24 hours in darkness and is independent of temperature (with variations in the range of 16° to 26° C); the lunar periodicity of valve opening in mollusks is maintained for a long time in an aquarium; the germinating capacity of seeds kept in darkness and at a constant temperature (in the range of -22° to 45° C) changes precisely according to the season. Under constant conditions, solar-diurnal rhythm is usually transformed into so-called circadian rhythm, with a period typical for each object, and differing somewhat from the 24-hour cycle. Circadian periodicity arises in organisms raised under constant conditions after a short-term change in those conditions, which indicates innate predisposition to such a rhythm. Thus, in Drosophila flies raised in darkness, rhythmic activity close to that of the normal arises after one 0.5 millisecond flash of light.

There are two points of view on the nature of biological rhythms: (1) Biological rhythms are based on strictly periodic physicochemical processes occurring in the organism—that is, on “biological clocks.” Changes in external conditions serve as time signals, which may shift the phases of the rhythm. Under constant conditions the rhythmicity is completely spontaneous, which is indicated by the noncoincidence of circadian rhythm with variations in geophysical factors. (2) The organism perceives cycles of pervading geophysical factors (geomagnetic field, cosmic rays, and so on). The internal system of time measurement, if it is present, plays an auxiliary role. Changes in illumination and temperature may shift the phases of a biological rhythm in relation to a geophysical cycle. Under the influence of constant but unnatural conditions, a regular shift of phase may arise.