Biological clocks

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.