''Your body temperature rises throughout the day and begins to decline around 7 or 8 o'clock at night,'' Dr. Campbell said. It falls to its lowest point about 5 or 5:30 A.M. then slowly starts to go up again. Similarly, melatonin begins to increase around 10 P.M. and makes people feel sleepy then falls off during the day.

On the second night, the subjects stayed awake in a dimly lighted room, reclining in a chair with a table over their laps. A thick black material was draped over their legs and fastened to their waists. Underneath this skirt, a knee pad with a fiber optic tube was attached to the back of their knees and a bright light was delivered through the tube for three hours.

Previous experiments with bright light delivered to the eyes showed that it is possible to advance or delay the body clock depending on when the light is given, Dr. Campbell said.

In the new experiment, subjects received light behind the knees at various times between midnight and noon, he said. For example, one man got the light treatment between 1 and 4 A.M. and another between 6 and 9 A.M.

Other subjects were put under the same dark skirt, kept awake the same amount of time and given the same instructions. But researchers did not turn on the light source. Neither group knew if it was getting the light treatment or not.

On the third and fourth nights, all subjects were told to stay in bed from midnight to noon and were allowed to sleep as their biological rhythms were measured. In similar experiments done with light to the eyes, body clocks were unstable on the third day and this was also the case with light to the knees. The fourth day was a surprise. For those treated with light, the timing of their minimum body temperature shifted by up to three hours. Those getting no light behind their knees experienced small but statistically insignificant changes in their bodily rhythms, Dr. Campbell said.

''This is the first demonstration that you can affect the human clock without going through the eyes,'' he said. ''We assume that somehow a message is getting from the back of the knee to the master clock'' in the brain.