Research has since confirmed that human beings have brain structures that govern REM sleep similar to those found in cats. Most warm-blooded animals, like mammals and birds, have periods of REM sleep. (Dolphins and many species of whales are notable exceptions.)

Image Michel Jouvet in 2005. Credit... Michel Depardieu/Inserm

The question of whether animals dream was asked millenniums ago by Aristotle and has since occurred to virtually anyone who has watched a pet twitching in a deep sleep. Dr. Jouvet, who cautioned that there was no way for people to know for certain whether animals have what we call dreams, made a startling discovery while observing their behavior with his fellow researchers.

When certain minute structures within a cat’s pons were destroyed in experiments, he found, the cat began to move during periods of REM sleep — movements suggestive of walking, stalking prey, grooming or acting aggressively — all without reacting to outside stimuli.

The cat was exhibiting what Dr. Jouvet called “oneiric behavior,” meaning behavior related to dreams, using a word derived from the Greek word for dream, “oneiros.” He theorized that what had been destroyed in the experiments were the cerebral structures that had inhibited the cat’s motion while it was sleeping, and that the cat had been reacting to its dreams physically.

“How can we wake a cat during paradoxical sleep and ask it questions?” Dr. Jouvet wrote. “We cannot, but the discovery and analysis of oneiric behavior would lead us to believe that cats do dream.”

In 1986, many years after Dr. Jouvet’s discovery of oneiric behavior in cats, a study by Carlos Schenck and Mark Mahowald at the Minnesota Regional Sleep Disorders Center in Minneapolis pointed to a similar phenomenon in humans. The study detailed a medical condition called REM behavior disorder, in which a sleeping person acts out violently in the throes of a vivid dream. The disorder can usually be controlled with a low dose of a sedative.