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.

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.)


References in periodicals archive ?
Our wellbeing is affected when there is a temporary mismatch between our external environment and this internal biological clock, for example when we travel across several time zones and experience 'jet lag.
A woman's biological clock begins ticking once she is 27 years of age and she experiences a good decline in the number of eggs by the age of 30.
Since humans are fundamentally diurnal creatures, staying awake at night can significantly disrupt all of the body's internal biological clocks.
The problem with this hypothesis was shown by Johnson and Rokas' study of the effect of changing codon usage on the simple biological clock found in single-celled cyanobacteria (blue-green algae) and a similar study of the more complex biological clock found in bread mold performed by a team led by Yi Liu that were published together.
They have found that a key protein that regulates the biological clock of mammals also regulates glucose production in the liver.
This biological clock or body clock however, does not measure time in minutes.
It adds further evidence for the role of the biological clock in autism.
Women still marry, bear children, and confront the life course stages based upon their marital status and biological clocks.
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.
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.

Full browser ?