In sleep science, the REM cycle is the stage of a person’s sleep that is characterized by rapid closed eye movements, very low or almost absent muscle tone (REM atonia, or sleep paralysis), and low voltage brain waves called theta waves. Typically, you will remember dreams that occurred during the REM cycle of sleep rather than during non-REM sleep. REM-stage dreams also tend to be much more vivid and interesting than NREM dreams, which are often mundane or may just be static images.
Research into the physiology of dreaming and REM sleep has led to some interesting discoveries: for one, it seems there are specific neural structures in the brainstem that generate the REM atonia, and possibly the theta waves, associated with dreaming in mammals. Experimental animal studies have pinpointed a structure in the brainstem called the locus coeruleus that may be responsible for inducing REM atonia: experimental animals with this brain structure destroyed move around in REM sleep as though they were acting out their dreams, while remaining oblivious to the outside world. Atonia, or lack of muscle tension in major muscle groups, is a distinctive feature of normal REM sleep, and the REM cycle can actually be divided into two phases based on the amount of muscle tension a sleeper exhibits. In the phasic part of REM sleep, small muscle groups will exhibit periodic bursts of activity, perhaps reflecting the sleeper’s actions in their dreams; in the tonic stage, muscle tension is almost entirely absent.
During an average night’s sleep (7-8 hours for an adult), your brain will typically enter the REM state 4 to 5 times in cycles that get longer toward the end of sleep. In total, adults spend about 90-120 minutes every night in REM sleep; that’s about 20-25% of an average 8 hours’ sleep. However, the duration and frequency of the REM cycle varies quite a bit between children and adults, with babies spending up to 80% of their time asleep in REM!
Because of this, some sleep researchers have speculated that REM cycling, and thus dreaming, might be essential for the healthy development of the human brain in infancy and childhood. Called the ontogenetic hypothesis of dreaming, this theory suggests that dreaming may have arisen as an evolutionary adaptation that enables the rapid development of neural connections and brain structures in neonates (i.e. infants). The corollary to this is that dreaming in adults may be an epiphenomenon with no evolutionary benefit. While it’s true that the amount of time we spend in REM cycles decreases as we age, some scientists also think dreams may help people maintain neuroplasticity and preserve learning abilities throughout life.
Another theory about the evolutionary function of REM cycles is that dreaming helps prepare us for life situations, and may be important for memory consolidation in both children and adults. For instance, our brains may dream as a means of filtering information we’ve been exposed to during the day or week, encoding important memories more deeply while discarding memories that haven’t been well encoded: the so-called “noise” of everyday life that would otherwise clog our neural networks. Alternatively, dreams may offer a sort of “practice space” for real life: by generating scenarios that may reflect real-life challenges, dreams may give animals and humans the chance to prepare in advance for the tasks of survival, such as hunting, gathering food, or fleeing from predators. If you’ve ever had a nightmare about running from some shadowy monster, the “practice mode” theory of dreams can be quite compelling!
Dreaming may even have the evolutionary function of waking us up. Though it sounds counterintuitive, the “Sentinel Hypothesis” suggests that the function of REM cycles might be to periodically wake birds and mammals up from a deeper sleep in order to scan their environment for predators. This is based on behavioral observations with birds and mammals in lab settings that show many species wake up for a brief time after the completion of a REM cycle. In this model, the function of REM atonia might be to allow the animal to quickly reenter sleep once it verifies there is no external threat. However, since waking after REM sleep is not a reliable occurrence in humans, this isn’t a totally satisfying explanation of why humans undergo REM sleep.
What is known is that all healthy human beings undergo REM cycles with the distinctive features of REM atonia, brain waves in the theta range, and of course rapid eye movement. Researchers still aren’t entirely sure about the purpose of these eye movements. Scientists once thought that they corresponded to what the sleeper was looking at in their dreamscape–the “scanning” hypothesis. However, it turns out the eye movements we experience during dreams often aren’t correlated with what we’re “looking” at in our dreams; in fact, sleep researchers have discovered that the eye movements of people undergoing a REM cycle often lack a focal point. Plus, congenitally blind people and fetuses in the womb also experience rapid eye movement, even though they are unlikely to have dreams with visual content. Instead, some researchers believe that rapid eye movements may lubricate the cornea and stimulate neural circuits in the brain that are not be activated when a person is awake, especially in young children where the central nervous system is rapidly developing.
There is still much we have to learn about the underlying physiology and purpose of the REM cycle and the epiphenomenon we call dreaming. Suffice to say for now that it seems to be a phenomenon that links all animals with complex nervous systems, and probably has a proven evolutionary benefit as well as a personal one to anyone who is intrigued by the experience and practice of dreaming.