Harvard researchers have located the body's tiny pacemaker that governs the rhythms of rest and activity in man and monkey, according to Dr. Martin Moore-Ede, who outlined the discovery in a paper given at the annual meeting of the American Association for the Advancement of Science.

The little double-knot of cells in the brain was known to exist in lower animals, but had never been located in primates or in man. The reason it has been missed is that the whole bundle of 20,000 cells is little more than one-hundredth of an inch long.

In experiments, Moore-Ede and Dr. Ralph Lydic monitored the rest, drinking, eating, and other activities of several monkeys, and plotted them on a chart. Monkeys eat, sleep, and drink on a more or less predictable daily schedule.

Then, using microsurgical techniques, Moore-Ede and Lydic destroyed the tiny pacemaker spot in the monkey's brain with an electrode needle.

After that, the pattern of rest and activity disintegrated. Instead of neat waves across the graph paper, there was a shotgun blast of dots. The same amount of waking, sleeping, eating, and drinking took place, but after destruction of the tiny pacemaker in the brain, all regularity was lost.

Similar experiments cannot be done directly on man. However, the two researchers and a colleague at Harvard, Dr. Charles Czeisler, have located the similar bundle of nerves in the human brain. They searched through medical records to find patients whose tumors or brain damage affected the spot where the pacemaker sits. In those records -- going back to 1898 -- the researchers found repeated reports of scattered, uncoordinated sleeping and eating habits in such patients.

"Now we hope to find a number of patients with these kinds of injuries and follow them for some time to record these things first hand," Moore-Ede said.

Scientists now believe that there are two chief pacemakers that govern the chemical and behavioral activities of the body. One, the one located by Moore-Ede and his colleagues, governs the sleep-wake cycle. The second governs the body's temperature rhythm, which normally rises and falls by about two degrees a day.

These two pacers affect one another, and set the pattern for the hundreds of other daily cycles that occur in the body. Blood pressure varies by up to 25 points over a day, cell division rates change by 1,200 percent daily, and several dozen chemicals in the blood and urine have their daily peaks and valleys. Many of these follow the daily cycling of one or the other of the body's pacers.

The result is that a person, chemically and biologically speaking, is a different creature at different times of day. Performance on virtually every kind of measure, from IQ tests to muscle strength, has been found to vary greatly with the body's daily cycles.

The pacer located by Moore-Ede and his colleagues is called the suprachiasmatic nucleus (SCN cells), and it lies directly over the spot where the two optic nerves cross. Its location is important because, in animals and even to some degree in man, daylight (seen through the eyes and passed down the optic nerve) resets the clock to a 24-hour rhythm every day. Otherwise, the body's clock tends to run a little slow, often going to a 25 1/2-hour day when a person is isolated from any time cues.

In the work on monkeys, the researchers found that when the nucleus of cells is cut off from the surrounding cells of the brain, including the optic nerves, the cells continue their rhythmic pattern of firing at different rates during different times of day.

This implies that, while the body's clock can be set by light or other influences, it runs on its own rhythm. The ticking of the clock -- the firing-rate of the SCN cells -- is governed by the internal chemistry of the cells themselves.

Moore-Ede said that his group will continue work on the SCN and its function, but now will also go in search of the second pacemaker, the one governing temperature and its attendant rhythms.