Biological Rhythms - How it works



Understanding Biological Rhythms

Among the many varieties of biological rhythm, the most well known are those relating to sleep and wakefulness, which are part of the circadian rhythm that we discuss later in this essay. Circadian, or daily, cycles are only one type of biological rhythm. Some rhythms take place on a cycle shorter than the length of a day, while others are based on a monthly or even an annual pattern.

Nor do all cycles involve sleep and wakefulness: menstruation, for instance, is a monthly cycle related to the sloughing off of the lining of the uterus, a reproductive organ found in most female mammals. Another biological rhythm is the beating of the heart, which, of course, takes place at very short intervals. Nonetheless, the circadian rhythm is the most universal of biological cycles, and it is the focus of our attention in this essay.

BIOLOGICAL CLOCKS.

In discussing the operation of biological rhythms, the term biological clock often is used. A biological clock is any sort of mechanism internal to an organism that governs its biological rhythms. One such mechanism, which we examine in the next section, is the pineal gland. Internal clocks operate independently of the environment but also are affected by changes in environmental conditions.

Examples of such alterations of conditions include a decrease (or increase) in the hours of available light due to a change of seasons or a change in time alteration due to rapid travel from west to east or north to south. In the latter instance, a condition known as jet lag—increasingly familiar to humans since the advent of regular air travel in the mid-twentieth century—may ensue.

The Pineal Gland

Governing human biological cycles—the "computer" that operates our biological clocks—is the pineal gland, a cone-shaped structure about the size of a pea located deep inside the brain. At one time, the great French philosopher and mathematician René Descartes (1596-1650) held that the pineal gland was actually the seat of the soul. Though it might seem absurd now that a respected thinker would seriously attempt to locate the soul in space, as though it were a physical object, Descartes's claim resulted from hours of painstaking dissection conducted on animals.

In searching for the human soul, Descartes sought that ineffable quality described some fifteen centuries earlier by the Roman emperor and philosopher Marcus Aurelius (121-180), who wrote, "This being of mine, whatever it really is, consists of a little flesh, a little breath, and the part which governs." As it turns out, the pineal gland is, in a sense, "the part which governs": it may not be the home of the soul (which, in any case, is not a question for science), but it does govern human circadian rhythms and thus has a powerful effect on the manner in which we experience the world.

MELATONIN.

The pineal gland secretes two hormones (molecules that send signals to the body), melatonin and serotonin. During the late 1990s, melatonin became a popular over-the-counter treatment for persons afflicted with sleep disorders, because it is believed that the hormone is associated with healthful sleep. Scientists do not fully understand the role that melatonin plays in the body, although it appears that it regulates a number of diurnal, or daily, events.

In addition, melatonin seems to serve the function of controlling fat production, which is one reason why good sleep is associated not only with a healthy lifestyle but also with a healthy physique. Many health specialists maintain that for adults there is a close link between a "spare tire" (that is, fat accumulation around the waist) and stress, lack of sleep, and low melatonin levels.

Among the many roles melatonin plays in the body is its job of regulating glucose levels in the blood, which, in turn, serve to govern the production of growth hormone, or somatotropin. Growth hormone is associated with the development of lean body mass, as opposed to fat, which is why athletes involved in the Olympics and other major sporting competitions sometimes have illegally "doped" with it as a means of increasing strength. It is not surprising, then, to learn that children—who clearly need and use more growth hormone and who also need more hours of sleep than adults—also have higher melatonin levels.

SEROTONIN.

Melatonin is not the only important hormone that is both secreted by the pineal gland and critical to the regulation of the body clock. Complementary to melatonin is serotonin, which is as important to waking functions as melatonin is to sleepiness. Like melatonin, serotonin serves several functions, including the regulation of attention.

Serotonin is among the substances responsible for the ability of a human with a healthily functioning brain to filter out background noise and sensory data. Thanks in part to serotonin, you are able to read this book without having your attention diverted by other sensory data around you: the voice of someone talking nearby, the sunlight or a bird singing outside, the hum of a light or a fan in the room.

By contrast, a person under the influence of the drug LSD (lysergic acid diethylamide) is not able to make those automatic filtering adjustments facilitated by serotonin. Instead, he or she is at the mercy of seemingly random intrusions of outside stimuli, such as the color of paint on a wall or the sound of music playing in the background. The secret of LSD's powerful hallucinatory effect can be attributed in part to the fact that it apparently mimics the chemistry of serotonin in the brain, "tricking" the brain into accepting the LSD itself as serotonin.

With regard to body clocks and biological rhythms, serotonin plays an even more vital governing role than does melatonin, since melatonin, in fact, is created by the chemical conversion of serotonin. On regular daily cycles the body converts serotonin to melatonin, thus influencing the organism to undergo a period of sleep. Then, as the sleeping period approaches its end, the body converts melatonin back into serotonin.

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