The Body Clock

No clock is perfect, however. When organisms are deprived of the external cues the world normally provides, such as light, they display a characteristic “free-running” period of not quite 24 hours. As a result, free-running animals drift slowly out of phase with the natural world. In experiments in which people are isolated for long periods of time, they continue to eat and sleep on regular, but increasingly out-of-phase, schedules. Such drift does not take place under normal circumstances, because external cues reset the clocks each day.

Light, particularly bright light, is believed to be the most powerful synchronizer of circadian rhythms. Recent studies on humans have shown that the amount of artificial indoor light to which people are exposed per day can resynchronize the body's cycle of sleep and wakefulness. People can inadvertently reset their body clocks to an undesired cycle by such activities as shielding morning light with shades and heavy curtains or by reading in bed at night by bright lamp light. Many organisms also make use of rhythmic variations in temperature or other sensory inputs to readjust their internal timers. When an internal clock's time is very different from the external time, complete resetting sometimes requires days. This phenomenon is well known to long-distance air travelers as jet lag. Apparently, biological clocks can exist in every cell and even in different parts of a cell. Hence, an isolated piece of tissue removed from an organism—for example, the eye of a sea slug—will maintain its own daily rhythm but will quickly adopt that of the whole organism when restored to it.

In the brains of most animals, a master clock appears to exist that communicates its timing signals chemically to the rest of the organism. For example, a brain removed from a moth pupa and exposed to an artificial sunrise of one time zone, then implanted into the abdomen of a headless pupa on a different time zone schedule, will cause the second pupa to emerge at the time of day appropriate to the disconnected brain floating in its abdomen. The clock in the brain triggers the release of a hormone that switches on all the complex behavior involved in pupal emergence. Scientists believe that the biological clock in humans is located in the hypothalamus, the part of the brain that regulates such basic drives as hunger, thirst, and sexual desire. The biological clock itself is believed to be a cluster of nerve cells called the suprachiasmatic nucleus. Melatonin, a hormone produced by the pineal gland in response to darkness, is thought to play a primary role in controlling the body's circadian rhythm. Recent studies have found that very low doses of melatonin, administered as a food supplement, can induce sleep, making the hormone potentially useful as a remedy for sleep disorders or jet lag.

Recent biochemical studies on fruit flies, as well as earlier research on bread mold, have revealed genes that play an important role in the biological clocks of these organisms. In bread mold, a gene known as freq has been shown to be integral to the mold's biological clock. In the fruit fly, a gene known as clock is turned on in the morning and activates two genes known as per (for period) and tim (for timeless). The proteins encoded by per and tim appear to interact together with light to govern the insect's biological clock. The same proteins govern the biological clocks of mice, raising the possibility that a similar system may exist in humans. Evidence suggests that a similar mechanism involving different proteins operates in such disparate organisms as cyanobacteria and plants. A fuller understanding of biological clocks could be important in many ways. One promising theory of aging, for example, is based on an observation that, in old age, the many separate, subordinate clocks in the body seem somehow to become less tightly coupled to the master clock in the brain. This lack of synchronization may contribute to many of the problems associated with aging.

A circadian rhythm is an approximate daily periodicity, a roughly-24-hour cycle in the biochemical, physiological or behavioral processes of living beings, including plants, animals, fungi and cyanobacteria. The term "circadian", coined by Franz Halberg,[1] comes from the Latin circa, "around", and diem or dies, "day", meaning literally "approximately one day." The formal study of biological temporal rhythms such as daily, tidal, weekly, seasonal, and annual rhythms, is called chronobiology. In a strict sense, circadian rhythms are endogenously generated, although they can be modulated by external cues, primarily daylight.

The first endogenous circadian oscillation was observed in the 1700s by the French scientist Jean-Jacques d'Ortous de Mairan who noticed that 24-hour patterns in the movement of the leaves of the plant Mimosa pudica continued even when the plants were isolated from external stimuli. In 1918 J. S. Szymanski showed that animals are capable of maintaining 24-hour activity patterns in the absence of external cues such as light and changes in temperature. The earliest known account of a circadian rhythm dates from the fourth century BC, when Androsthenes, in descriptions of the marches of Alexander the Great, described diurnal leaf movements of the tamarind tree.

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