Physical activity may play an unexpected role in keeping mammals synchronized to the 24-hour day, researchers reported.

Their findings question the conventional view that in most mammals, the brain sets the 24-hour biological clock chiefly by monitoring light and darkness in the environment.The circadian clock governs many body processes, and in humans, disruptions lead to such conditions as jet lag and some cases of insomnia.

In Thursday's issue of the British journal Nature, researchers said they found that onset of darkness failed to exert its usual clock-setting effect in hamsters if the animals were immobilized.

So the real clock-setting mechanism may be the physical activity that darkness usually causes in hamsters, suggested Fred Turek of Northwestern University and O. Van Reeth of the Free University of Brussels in Belgium.

The paper "is going to generate considerable controversy over the next year," commented Dale Edgar, director of the chronobiology and behavioral pharmacology laboratory at the Sleep Research Center of the Stanford University School of Medicine.

But he said its relevance to humans was not clear. The experiment has only been done in hamsters, he stressed. He also said that in humans, social interaction and the scheduling of meals strongly influence the setting of the clock.

He said previous research, including his own, has shown that the scheduling of physical activity can reset the clock in animals. Nobody knows whether that strategy can affect the human clock, he said.

In the Nature paper, Turek and van Reeth reported that unrestrained hamsters showed the usual changes in their 24-hour activity schedules in response to darkness or injections of the sleeping-pill drug triazolam.

In contrast to its effect in humans, triazolam increases physical activity in hamsters, as does darkness. Either treatment can advance or delay the clock setting, depending on the time in the 24-hour cycle at which it is given, researchers said.

But when animals were immobilized, the effect of darkness was eliminated and the impact of triazolam was largely absent, they reported. Hamsters were immobilized by placing them in a transparent tube.