In 1951, Eugene Aserinsky, a physiologist at the University of Chicago, placed electrodes on the scalp of his 8-year-old son, Armond, before putting him to bed. Then the scientist recorded the electrical activity in the boy’s facial muscles.

Hours later, the pens quivering across the paper swung wildly. To judge from the chart, it seemed as if Armond were awake, but he wasn’t. Aserinsky had discovered REM sleep.

He and other researchers learned that during this state, the brain shifts from low-frequency to high-frequency electrical waves, like those produced in waking hours.

Almost all mammals experience REM sleep, but researchers still debate why it exists. Now, a team of U.S. and Russian researchers reported that fur seals may provide a key clue.

While they swim, fur seals switch off REM sleep. It returns when they come back to land — a pattern never seen before.

Study co-author Jerome M. Siegel, a sleep expert at the University of California, Los Angeles, said that provides evidence that our brains switch to REM sleep to generate heat. “REM sleep is like shivering for the brain,” he said.

His team proposes that the brain cools during slow-wave sleep. To keep the brain from getting too cold, the brain periodically unleashes a torrent of activity. Oxygen-rich blood flows into the brain to fuel the activity, warming the brain.

“It keeps the brain temperature within a functional limit by cycling on and off,” Siegel said.

This could account for why dolphins don’t experience REM sleep — and why seals don’t as they swim. These mammals have evolved a half-brain style of sleeping, perhaps as a way to remain alert enough to avoid predators and drowning.