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the branch of biology that studies the conditions under which biological rhythms arise, as well as the nature, regularities, and significance of biological rhythms (seeBIOLOGICAL RHYTHMS).

Chronobiology investigates rhythmic processes at different levels of organization: noncellular systems, the cell, unicellular organisms, cell and tissue cultures, multicellular animals and plants, and populations of organisms. As a branch of biology, it establishes the laws governing periodically repeating biological processes and the behavior of different biological systems over time. It is closely allied with physiology, biochemistry, ciophysics, ecology, and other natural sciences.

Biological rhythms are very common in living nature. They are endogenous in origin and depend on rhythmic changes in the environment (photoperiods, thermoperiods, changes in barometric pressure, fluctuations of the earth’s electromagnetic field). The interaction of the biological rhythms with one another and with the periodically changing environment creates the temporal organization of biological systems, underlies the adaptation of organisms, and unifies animate and inanimate nature. Biological rhythms reflect the processes that regulate the functions of organisms, regardless of the length of the period or the frequency of occurrence (daily, monthly, seasonal, annual).

The idea that rhythmic processes occur in nature and in the human body can be found in the works of ancient philosophers (Heraclitus, Plato, Aristotle) and in medieval and Renaissance works (F. Bacon, Tycho Brahe, J. Kepler). Biological rhythms were first observed scientifically by the French astronomer J. J. De Mairan (1729), who detected daily periodicity in the leaf movements of plants. This phenomenon was later studied by C. Darwin (1880) and a number of 19th-century botanists. As early as the 18th century, C. Linnaeus proposed flower clocks, based on the ability of the flowers of different plants to open and close at certain times of the day. The rhythms of leaf movement were thoroughly investigated in the 1930’s by the Dutch botanist A. Kleinhoonte and by the German scientist E. Bünning. In 1920 the American scientists W. W. Garner and H. A. Allard discovered photoperiodism in plants; its mechanisms were subsequently found to be closely related to biological rhythms.

In the 19th century, biological rhythms were also recorded in animals and humans. The first studies of photoperiodism in animals, both vertebrates and invertebrates, were conducted in the 1920’s.

Russian and Soviet scientists have made substantial contributions to the study of biological rhythms. I. M. Sechenov, I. P. Pavlov, and V. M. Bekhterev conducted research on the perception of time by animals and man. N. E. Vvedenskii and A. A. Ukhtomskii scientifically explained the regularities of rhythmic influences on the cell and the phenomenon of cell “assimilation” of external rhythms. V. I. Vernadskii was the first to regard the biosphere as a system organized both in space and in time. K. M. Bykov conducted a series of studies on biological rhythms in man and animals, while A. S. Danilevskii conducted fruitful research on photoperiodism in insects. A. L. Chizhevskii, the founder of heliobiology, studied the influence of solar rhythms on living organisms. The role of biological rhythms in regulating functions and changes therein during spaceflight was elucidated by V. V. Parin.

Others who have contributed significantly to chronobiology in the 20th century include A. Jores (Federal Republic of Germany), J. Möllerstrom and E. Forsgren (Sweden), J. W. Hastings and F. A. Brown (United States), and J. Cloudsley-Thompson and J. E. Harker (Great Britain).

In the early 1930’s Bünning (Germany) proposed that biological rhythms are endogenous. J. Aschoff (Federal Republic of Germany) conducted basic research on how environmental factors affect biological rhythms, including those in man, and in 1951 coined the term Zeitgeber (“time giver” or “time “cue”) to designate the factor that synchronizes biological rhythms. F. Halberg (United States) formulated (1959) the concept of circadian rhythms and advanced the idea of the temporal coordination of physiological functions. He is credited with introducing mathematical methods of processing data into chronobiology and the use of computers. He also established that sensitivity to injurious factors varies with the time of day.

Elucidation of the regularities of the temporal course of biological processes has contributed to the development of various branches of biology and has proved to be of great practical significance. For example, a knowledge of photoperiodism is important for agriculture. Medicine makes use of chronobiological data in the diagnosis and treatment of certain diseases. Important areas of research in chronobiology include the study of the nature and mechanism of various biological rhythms, the influence of external factors on biological rhythms, the role of biological rhythms in adaptation to the environment, the function of biological rhythms in man’s work and in the development of disease, and various problems related to space biology and medicine.

In the USSR, research on biological rhythms is conducted at the I. P. Pavlov Institute of Physiology (Leningrad), the Institute of Biophysics (Pushchino, Moscow Oblast), the Institute of Biomedical Problems (Moscow), Moscow State University, the Second Moscow Medical Institute, the A. V. Vishnevskii Institute of Surgery (Moscow), the K. A. Timiriazev Moscow Agricultural Academy, the Siberian branch of the Academy of Sciences of the USSR, and elsewhere.

The major foreign research centers include the universities of Minnesota and Arkansas and Stanford University in the United States, the universities of Tübingen and Göttingen and the Max Planck Institute of the Physiology of Behavior in the Federal Republic of Germany, and the University of Manchester in Great Britain.

The Society for Biological Rhythm Research was organized in 1937, at the first conference on chronobiology in Ronneby, Sweden. In 1971, at the conference held in Little Rock, Ark., it was renamed the International Society for Chronobiology; it now has more than 300 members from 30 countries. An international symposium on biological clocks was held in Cold Spring Harbor, N.Y., in 1960.

In the USSR chronobiological topics were discussed at the all-Union symposia Biological Rhythms in the Mechanisms of Compensation of Impaired Functions (Moscow, 1973) and Circadian Rhythms of Humans and Animals (Frunze, 1975).

The results of research in chronobiology are published in specialized journals, such as the International Journal of Chronobiology (London; since 1973), Chronobiologia (Milan; 1974), and the Journal of Interdisciplinary Cycle Research (Amsterdam; 1970).


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