Feel like hibernating? Blame the brain.

For many people, a nap by the fire makes even the worst cold weather bearable.

For animals, however, surviving a winter often requires spending days on end at unusually cool body temperatures in a torpid condition.

What prompts animals to begin and end hibernation has long interested scientists. They knew that animals hibernate at predictable times, even under unusual lighting and temperature conditions. During that period, animals boost their body temperature to normal and rouse themselves for a day or so every 2 to 14 days. Yet the location of the internal clock thought to control these hibernation schedules remained elusive.

Now, Norman F. Ruby of Stanford University and his colleagues report that the brain's hypothalamic suprachiasmatic nucleus (SCN)-best known for its role in governing the body's daily rhythms (SN: 12/3/88, p. 366)-also regulates hibernation.

Ruby and his coworkers destroyed the SCN of eight golden-mantled ground squirrels (Spermophilus lateralis), which normally hibernate for 5 to 7 months a year. For 2 1/2 years, the team compared in the laboratory the hibernating habits of these animals to those of six intact squirrels.

The animals lacking an SCN roused themselves more often during their hibernation than the undamaged squirrels did, the team reports in the Sept. 3 Proceedings of the National Academy of Sciences.

In addition, although they initiated hibernation at the normal time, four of the altered animals hibernated for a month after the intact creatures emerged.

The other four SCN-less squirrels cycled through bouts of torpor almost continuously throughout the study.

"That's phenomenal," said Brian M. Barnes of the University of Alaska, Fairbanks.

The continual hibernators may have suffered injuries to a region of the SCN that remained unharmed in the other four, the team speculates. The animals could also have suffered from injuries close to the SCN, adds Barnes.

Removing the SCN disrupted normal annual weight-gain cycles, Ruby says. The extended hibernators maintained lower weights than the other SCN-less squirrels, which had lower weights than the intact squirrels.

Loss of the SCN also disrupted the squirrels' circadian rhythms of activity; however, this had little influence on hibernation patterns, the authors assert.