Rapid Eye Movement Sleep, or REM, is one of the four stages of sleep that make up our nightly sleep cycle. It includes vivid, complex dreaming, heightened brain activity, and the frequent eye movements that give REM its name.
While all four stages of sleep have their own distinctive characteristics, REM sleep is particularly unlike the others. Stages 1-3 of sleep, collectively known as non-REM (or NREM) sleep, are progressively deeper phases of sleep, characterized by often slower and more synchronized brain wave activity.
During REM sleep, the brain becomes very active, with brain wave frequencies that closely resemble the activity of a waking brain. Because of its striking similarity to a waking, “thinking” brain, it is sometimes referred to as paradoxical sleep.
In a typical night of sleep, this particular phase occurs at the end of the first sleep cycle, after the sleeper has moved through all stages of NREM sleep. After this first phase, the sleep cycle ends and a new sleep cycle begins. In each successive sleep cycle throughout the night, we spend progressively more time in REM sleep, as periods of REM within each sleep cycle grow longer.
Time spent in REM sleep changes over the human lifespan. Infants and very young children spend significant amounts of time in REM, about 50 percent of their overall sleep time. Adults typically spend about 20-25 percent of their total sleep time in REM.
REM is an active stage for dreaming. Though we don’t know the reason for the frequent eye movements that occur during this stage, they may be associated with the visual imagery of the dreams that happen during REM. The dreams that take place tend to be more vivid and complicated—and more likely to be remembered—than dreams that take place in other sleep stages. Much of what we consider typical dreaming—dreams populated with characters, events, and action, with visual and emotional texture and depth—appears to unfold during this stage.
There are other physiological changes that occur that make it distinctive from the stages of NREM sleep. Breathing, blood flow, and heart rate, which slow during the progressively deeper stages of NREM sleep, increase during REM, returning closer to waking levels.
In REM sleep, the body enters a state of temporary paralysis, known as REM atonia. The major muscle groups are paralyzed, with muscle reflexes and tone suspended. Scientists don’t know precisely why this temporary paralysis occurs, but it’s believed to be a protective measure taken by the body to avoid acting out physically in response to the dreams that are common in REM.
Certain sleep disorders, including narcolepsy and REM sleep behavior disorder, involve the absence of this REM paralysis that keeps a sleeper still. Without REM atonia in effect, sleepers often respond physically to their dreams, sometimes aggressively so. Sleepers may thrash, kick, grab, jump and run from bed, as well as yell or scream.
Reasons for REM?
We don’t fully understand the underlying reason for REM sleep, but decades of study strongly suggest that it delivers important developmental and restorative functions for the brain. In infants, it appears to play a critical role in neurological development, particularly in the growth and development of communication networks within the brain and the central nervous system. Does it continue to have a purpose for adults? Research suggests several possibilities. In addition to its role in some of our most active dreaming, it may also aid:
- Memory consolidation, especially for procedural and spatial memories
- Emotional processing, particularly for difficult, emotionally-laden experiences in waking life
- Clearing out the clutter of the waking day’s accumulated learning and information—a kind of refresh button for the mind
Dreams occur during this sleep stage. To learn more about dreaming, download my free Dreams ebook.