Any sleep tracker will show you that slumber is far from a passive affair. And no stage of sleep demonstrates that better than rapid eye movement, or REM, commonly called dream sleep.

“It’s also called paradoxical sleep or active sleep, because REM sleep is actually very close to being awake,” said Dr. Rajkumar Dasgupta, a sleep medicine and pulmonary specialist at the Keck School of Medicine of the University of Southern California.

Before scientists discovered REM sleep in the 1950s, it wasn’t clear that much of anything was happening in the brain at night. Researchers today, however, understand sleep as a highly active process composed of very different types of rest — including REM, which in some ways doesn’t seem like rest at all.

While the body typically remains “off” during REM sleep, the brain is very much “on.” It’s generating vivid dreams, as well as synthesizing memories and knowledge. Scientists are still working to unravel exactly how this strange state of consciousness works.

“It is fair to say that there is a lot left to learn about REM sleep,” Dr. Dasgupta said. But from what researchers do understand, REM is critical to our emotional health and brain function — and potentially even our longevity.