Matthew Zeigler, age 15, of Longview, Wash., for his question:

WHAT IS THE BIOLOGICAL CLOCK?

Biological clock is the popular name given to a mysterious timing system that operates in humans, animals and plants. It keeps accurate time of days, weeks, months and years. It also times the activities of living things to keep them in harmony with the regular changes in their surroundings.

Biological clocks in man work on schedules essential to life and health. Many vital body processes are programmed for 24 hour rhythms. The functions of cells, glands, kidneys, liver and nervous system are all coordinated with each other and with the day night rhythm of the environment.

Each body function is timed to operate when it can do the greatest good for the entire body.

Throughout the day, the rate at which the body processes work gradually changes. The temperature of the body is about two degrees lower during the nighttime rest period, for example, than it is during the active daylight hours.

Biological clocks are based on a day night cycle. The daily rhythms are called circadian rhythms because they occur about every 24 hours. "Circadian" comes from the Latin words that mean "about a day."

Some scientists say biological clocks are set by the natural rhythms of the earth's electromagnetic or gravitational forces. Still others suggest that forces both inside and outside the body are necessary to keep the clocks accurate.

All living things seem to respond to rhythmic changes. The response in each case seems to be timed to give the best advantage to that particular species.

Biological clocks set the schedules for yearly rhythms in living things. They control the sprouting of seeds, hibernation and the migration of birds and other animals.

Better knowledge of biological clocks and the rhythms they control will help benefit man.

You can quickly become aware of the biological timing system when you fly in a jet airplane to a different time zone. For example, a flight from Los Angeles to New York that leaves Los Angeles at night will reach its destination just as New Yorkers are starting their day.

Your rhythmic system will still be working on Los Angeles time. By New York time, you will have insomnia at night and be sleepy during the day. Your biological clock will eventually reset itself, but it may take several days.

As a result of having your biological clock thrown off of its schedule, your body functions are out of rhythm, your efficiency drops and you feel tired.

This change of rhythm is sometimes called "jet lag" or "jet exhaustion."