No clock is perfect, however. When organisms are deprived of the cues the world normally provides, 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 a clock’s error becomes large, 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. In hamsters, experiments have shown a master biological clock to be located in the hypothalamus.
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 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 discovered 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, two proteins-encoded by two genes known as per (for period) and tim (for timeless)-appear to interact together with light to govern the insect’s biological clock. How the fruit fly’s clock mechanism relates to that of humans is not clear because neither tim nor per has been found in mammals.
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.