By Peter M. Vishton, PhD, William & Mary

Edited by Kate Findley and proofread by Angela Shoemaker, The Great Courses Daily

Have you ever wondered why you still feel groggy after getting eight hours of sleep? Professor Vishton explains why feeling rested boils down to more than just the amount of sleep you get. It’s also about the type of sleep—specifically, getting enough REM sleep.

What Is REM Sleep?

During REM sleep, while the brain becomes very active, the muscles in most of the body become deeply relaxed. Throughout the day, even when you’re just sitting still, your muscles maintain a certain level of tone. 

It’s rigidity that’s produced by a relatively slow but steady stream of neural impulses delivered from your brain, via your spinal cord, to your peripheral nerves. As you enter the REM stage of sleep, however, those signals drop off. The muscles become remarkably inactive, almost paralyzed.

The muscles that control the movements of the eyes are an exception to this. The eyes become very active, darting in all directions, as if rapidly scanning something. 

The eyes are still closed, but you can see the eyeballs moving behind the lids if you look. These rapid eye movements are the feature that gives this sleep stage its name: REM. If you awaken someone when they’re exhibiting these sleep characteristics, he will always tell you that he was having a dream.

Other Sleep Phase Characteristics

Other sleep phases have particular characteristics associated with them as well. In stage 2, people often experience hypnic jerking. 

Many people report a periodic experience of a momentary dream of falling. They jerk briefly awake with a bit of an adrenaline rush. 

In early stage 3, people can sleepwalk. By the way, it’s fine to wake up someone who’s sleepwalking. 

It’s better to wake them to avoid injuries. Usually sleepwalking only involves walking into a door or stubbing a toe, but it’s not unheard of for someone to leave their house and even to drive a car while sleepwalking.

In stage 4 sleep, many children experience night terrors, in which they begin screaming out, seemingly in great fear, in the middle of the night. One reassuring consequence of the fact that stage 4 sleep is such a deep sleep is that most children who experience night terrors have no idea that they’ve done so. 

Why REM Matters

While all of these sleep stages have particular processes associated with them, there’s a wealth of evidence that your brain does its most important work that supports regular brain function during REM sleep—the dreaming stage. One example is memory. 

You tend to dream about the things that have happened to you during the recent past—most frequently about things that happened during the previous day. Early in the evening, those dreams will tend to be very literal. 

If you went to the park with your dog and your mom, a dream early in your sleep time is likely to be about that trip to the park with your dog and your mom. Later in the evening, the dreams tend to become more abstract in nature. A dream later during the night might be about travels to distant lands taken with large animals and family members.

This replay and abstraction process is critical to optimal memory as well as high levels of creativity and problem-solving. We encode things into our long-term memory as we experience them during the day, but those memories seem to be consolidated in an important way during the following night of REM sleep. 

When We Don’t Dream

If you prevent someone from getting REM sleep—even if you allow them to get a lot of non-REM sleep—detailed memory for experiences and new things that they learned during that day will tend to suffer. Cognitive function also generally declines, according to a wide range of cognitive function measures.

In a sleep lab, this REM deprivation is accomplished by letting someone sleep and identifying when they enter that REM sleep phase. As soon as they do, the researcher wakes the participant up, talks to him for a minute, and then lets him drift off to sleep again. 

Unfortunately for the participant, he can’t just jump back into his sleep cycle right at that REM stage; he has to start again at stage 1. About 90 minutes later, when he’s about to enter the REM stage again, the experimenter wakes the participant up.

If this happens all night, the participant’s brain won’t work so well in the morning—reaction time gets slower, short-term memory function drops, creativity is worsened, and emotional regulation is reduced. 

Angry outbursts and mood swings are far more likely for someone who’s been prevented from getting their normal REM sleep—even while getting eight hours of sleep. Your brain needs to dream just like it needs nutritious food.

This article was edited by Kate Findley, Writer for The Great Courses Daily, and proofread by Angela Shoemaker, Proofreader and Copy Editor for The Great Courses Daily.

Peter M. Vishton is Associate Professor of Psychology at William & Mary. He earned his PhD in Psychology and Cognitive Science from Cornell University. Before joining the faculty of William & Mary, he taught at Northwestern University and served as the program director for developmental and learning sciences at the National Science Foundation.