On a December evening 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 retired to another room to watch a row of pens quiver across a rolling sheet of paper, recording the electrical activity in the boy’s facial muscles.

Hours later, the pens started to swing wildly. To judge from the chart, it seemed as if Armond were awake, his eyes darting about the room. But when Aserinsky looked in on him, his son was fast asleep.

Aserinsky had discovered R.E.M. sleep.

Eventually 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. When Aserinsky woke his subjects from R.E.M. sleep, they often reported vivid dreams.

Almost all mammals experience R.E.M. sleep, but even today researchers debate why it exists. On Thursday, a team of American and Russian researchers reported that fur seals may provide an important clue.