Biological Clocks

Also found in: Dictionary, Thesaurus, Medical.

Biological clocks

Self-sustained circadian (approximately 24-hour) rhythms regulating daily activities such as sleep and wakefulness were described as early as 1729. By the midtwentieth century it had become clear that the period of self-sustained (free-running) oscillations usually does not match that of the Earth's rotation (environmental cycle), therefore the expression “approximately 24 hours.” Moreover, the free-running period varies among species and also somewhat from one individual to another. Circadian rhythmicity is often referred to as the biological clock. See Photoperiodism

Almost all organisms display circadian rhythms, indicating an evolutionary benefit, most likely facilitating adaptation to the cyclic nature of the environment. Physiological processes that occur with a circadian rhythm range from conidiation (spore production) in the bread mold, Neurospora crassa, and leaf movements in plants to rest-activity behavior in animals. Despite the diversity of these phenomena, the basic properties of the rhythms are the same—they synchronize to environmental cues, predominantly light, but are maintained in the absence of such cues, and they display a constant periodicity over a wide temperature range.

In humans, circadian rhythmicity is manifested in the form of sleep-wake cycles, and control of body temperature, blood pressure, heart rate, and release of many endocrine hormones. It is increasingly apparent that temporal ordering is a fundamental aspect of physiological processes. In fact, several disorders such as asthma, stroke, and myocardial infarction also tend to occur more frequently at certain times of the day. Awareness of circadian control has led to the concept of chronotherapeutics, which advocates drug delivery timed to the host's circadian rhythms.

In mammals the “master clock” controlling circadian rhythms is located in the hypothalamus, within a small group of neurons called the suprachiasmatic nucleus. Available data suggest that the suprachiasmatic nucleus transmits signals in the form of humoral factors as well as neural connections. For many years the suprachiasmatic nucleus was thought to be the only site of a clock in mammals. This was in contrast to several other vertebrates where clocks were known to be present in the pineal gland and the eye as well. However, it is now clear that the mammalian eye also contains an oscillator (something that generates an approximately 24-h cycle) whose activity can be assayed by measuring melatonin release in isolated retinas. See Nervous system (invertebrate), Nervous system (vertebrate)

The genetic basis of circadian rhythms was established through the identification of altered circadian patterns that were inherited. Such mutants were found first in Drosophila and then in Neurospora in the early 1970s. In addition, there is now an impetus to identify circadian abnormalities or naturally occurring variations in human populations. For instance, the difference between people that wake up and function most effectively in the early morning hours as opposed to those who prefer to sleep late into the morning may well lie in polymorphisms within clock genes.

It is now known that a feedback loop composed of cycling gene products that influence their own synthesis underlies overt rhythms in at least three organisms (Drosophila, Neurospora, and cyanobacteria) and most likely in a fourth (mammals). Similar feedback loops have also been found in plants, although it is not clear that they are part of the clock.

McGraw-Hill Concise Encyclopedia of Bioscience. © 2002 by The McGraw-Hill Companies, Inc.
The following article is from The Great Soviet Encyclopedia (1979). It might be outdated or ideologically biased.

Biological Clocks


a conventional term denoting the ability of a live organism to orient itself in time.

The basis of biological clocks is the strict periodicity of physicochemical processes occurring in the cells. The speed of these processes changes regularly. The rhythm of these changes is genetically linked to natural selection and is connected with cyclic changes in geophysical factors. Several chemical, physical, and mathematical models of biological clocks have been proposed. Some researchers believe that the basis of biological clocks is the ability of organisms to perceive cyclic fluctuations in pervasive geophysical factors (daily and seasonal periodicity of the electrical and magnetic fields of the earth, solar and cosmic radiation, and others). A system of time measurement connected with biological clocks and permitting the reckoning of any intervals (conditioned reflex for telling time) develops in animals.


Emme, A. M. Biologicheskie chasy. Novosibirsk, 1967.
Marteka, V. Bionika. Moscow, 1967. Pages 11–31. (Translated from English.)


The Great Soviet Encyclopedia, 3rd Edition (1970-1979). © 2010 The Gale Group, Inc. All rights reserved.
References in periodicals archive ?
However, current approaches to these problems have significant limitations, as a single therapy can't address the disruptions that occur in all biological clocks. For example, when used incorrectly, light therapy can even aggravate the situation," stated Dr.
Our study shows the biological clock determines how well these hormones work," said Marc Montminy, of the Salk Institute for Biological Studies, who led the study.
Our dependence on clocks prevents us from noticing our inner time on our biological clock. "In fact each of our cells contains a mechanism for measuring time and our central clock consists of a pair of ganglia the size of a grain of rice called the suprachiasmatic nuclei, located in each hemisphere of the brain a couple of centimeters behind the bridge of the nose...
But dispelling existing myths about men's roles in pregnancy is not easy in a society where the words "male" and "biological clock" aren't typically used in the same sentence.
The research seems to confirm men have biological clocks as well as women.
Women still marry, bear children, and confront the life course stages based upon their marital status and biological clocks. Today they do so, however, in an equal opportunity society.
Biological clocks have been identified in many organisms, including human, birds, fish, end reptile; insects, crustaceans, and mollusks; flowering plants, weeds, and trees; end even in single-celled forms, such as algae and fungi (Charles Weitz 1995).
Since his earliest LED installations, in 1987, Tatsuo Miyajima has been reshaping the timeless and pristine white cube into a black abyss governed by the pulse of our biological clocks. While the Japanese artist is best known for orchestrating digital LED counters into richly varied arrangements strewn across the floor, installed in geo metric patterns on walls, even placed on little robotic cars-the works in his recent installation "Totality of Life" span a wider range of media and incorporate a certain humanist dimension that his earlier installations lacked.
MEN'S biological clocks tick away the same as women's and their fertility declines after they reach 24, scientists have found.
Researchers have found that the hormone melatonin resets teenagers' biological clocks, in effect juicing them to stay up later--and sleep in later.
New research shows teens' biological clocks are at odds with most high school schedules--and it's affecting these students' ability to learn.
As reported in Science in June, researchers were repeatedly able to readjust the biological clocks of 14 men (age 18 to 24) by exposing them to light (equal to sunlight in brightness) for three five-hour sessions at various stages of their circadian cycles over the course of three days.

Full browser ?