THOUGH OUR BODIES are inert during sleep, there’s a lot going on in the brain. If you’ve ever seen a person or pet sleeping, you may have noticed that their eyeballs are rolling around rapidly. This happens during the Rapid Eye Movement (REM) stage of the sleep cycle.

Almost all mammals experience REM sleep, and babies spend a lot more time in the REM stage than adults do. We dream during REM, but does it serve a biological purpose? Believe it or not, sleep is still one of the body’s most mysterious functions.

The Sleep Cycle

Sleep experts generally recognize five stages of sleep. Stage 1 is light sleep, a drifting-off period. Though many people see images or are aware of fragmented thoughts during this stage, we don’t dream in Stage 1 sleep. Stage 1 lasts for about 10 to 20 minutes, during which time you may experience the sensation of falling and then jerking awake. There’s a name for that! It’s called hypnic myoclonia.

In stages 2, 3 and 4, brain waves become progressively slower. Stage 2 accounts for about half the total sleep cycle. Stages 3 and 4 are considered deep sleep. After Stage 4, we transition to REM. The full sleep cycle lasts 90 minutes, on average, and then starts back at Stage 1.

REM vs Non-REM Sleep

The REM stage only accounts for about 20 percent of our total sleep time. With each subsequent sleep cycle, REM sleep gets longer while deep sleep gets shorter.

During REM sleep, the functions of the autonomic nervous system, including heart rate, blood pressure and breathing, become faster than normal. We also experience a temporary paralysis, which is why you may sometimes have a “lucid dream,” a state in which you are aware that you’re dreaming, but are unable to move.

An electroencephalogram (EEG) is a test that tracks brain waves and reports the results to a computer. These tests show that brain activity during REM sleep, which occurs when we’re dreaming, is almost equal to activity while awake. Yet during non-REM sleep, brain activity is much slower.

What Is the Purpose of REM Sleep?

Despite numerous studies, scientists haven’t been able to pinpoint the biological purpose of REM sleep, though there are some prevalent theories. Several studies found that when lab rats were deprived of REM but allowed to have non-REM sleep, they lost much of their long-term memory function in four days and died within five weeks.

These studies concluded that REM stage plays a critical role in something called brain plasticity. Brain plasticity, sometimes called neuroplasticity, is the brain’s ability to reorganize and establish connections between various parts of the nervous system. It’s the mechanism that allows us to learn new things and remember things we’ve previously learned.

Other theories regarding the role of REM:

• It helps us derive meaning from our environment
• It evolved from a prehistoric defense mechanism, the “faint response”
• It allows us to resolve emotional issues

Though scientists may not yet agree on why we need REM sleep, it’s clear that adequate sleep is vital to our cognitive function and overall health. Adults need seven to eight hours of quality sleep per night, while children need considerably more. Given the role of sleep in learning and memory, it’s easy to see why it’s important to prioritize getting enough sleep each night.