REM Sleep

The Significance of Rapid Eye Movement

REM is something of a household word as scientific terms go. Most people can tell you that its letters stand for “Rapid Eye Movement,” referring to the way your eyes dart around underneath your eyelids during certain phases of sleep. Many people will also remember that REM is closely associated with dreaming.

Still, you might wonder what all the fuss is about. What is it about these eye movements that make them so significant and compelling in the world of sleep science?

To understand, you almost need to consider REM as something set apart from sleep rather than as a mere feature of it. Scientists categorize sleep into two basic states called REM sleep (when rapid eye movement takes place) and non-REM sleep (when the eyes remain still). When you are in REM sleep, your body and brain are doing something radically different from what goes on in ordinary non-REM sleep.

In fact, REM sleep is different enough that it is regularly put on par with wakefulness and non-REM sleep as one of the three basic behavioral states. (Think of that: At any given time, you’re either awake, asleep, or in REM.) The British sleep researcher, Jim Horne argues that REM sleep is in many ways more like a form of wakefulness than sleep and wonders if it shouldn’t be called “REM wakefulness.”

REM essentially went undiscovered until 1952 — recent in terms of other scientific breakthroughs. You might say that it had been hiding in plain sight; the eye movements are super easy to spot if you know to look for them. But its discovery marked a real milestone in science and medicine, completely revolutionizing our understanding of what sleep is and opening new perspectives on how and why we dream. Most importantly, REM put an end to the idea of sleep as a period of time when we just shut down and nothing happens.

Something is very definitely happening during REM sleep. So let’s kick off our exploration of REM with an obvious question: What the heck is it?

And Now For Something Completely Different…

Picture yourself on any given night, curled up, asleep in bed. It’s been a little less than an hour and a half since you turned out the light, climbed in and dozed off. You’ve made your way from the initial stage of drowsy half-sleep, to a solid but light sleep, then all the way into a third stage of total, zonked out deep sleep.

As you’ve descended through each stage, your body and mind have progressively slowed down, so that by now, your body is totally relaxed and your brain is thoroughly disengaged. Your blood pressure and heart rate are lowered and your breathing is nice and slow and deep. Your brain waves, which positively crackle with activity during waking hours, are now calmed and flowing in a series of slow, synchronous waves.

But now a very curious transition is about to take place: Suddenly, your body starts speeding up again. Your breathing quickens and grows somewhat irregular, your heart rate and blood pressure increase, and your eyes begin darting about rapidly underneath closed eyelids. You’ve just entered REM sleep. The rapid eye movements are, of course, its hallmark, but as we’ll see in a moment, there’s a lot more going on here than just eye movements.

Through it all, your body remains perfectly still; if someone saw you lying there, you would appear to be absolutely sound asleep. But inside your sleeping body, your brain has, in a sense, woken up. In a matter of moments, it has gone from a kind of dormancy to a state of high activity. Neurons have sprung into action, firing off a storm of signals in brainwave patterns that are remarkably similar to that of wakefulness. Some parts of the brain are even more active now than when you are actually awake.

All this neural activity usually means one thing: You are having a dream. Your consciousness has reawakened into a self-generated realm of virtual reality. In it, you might find yourself flying through the air like a bird, running away (in slow motion) from a shadowy monster, shooting the breeze with deceased relatives, or back in school, hopelessly late for your final exam.

We are known to dream throughout the night in every phase of sleep. But REM sleep seems to be a particularly fertile ground for dreams: When you’re in REM, there’s an 80 to 90% chance you’re dreaming, compared to only a 30% chance while in non-REM. And REM dreams tend to be much more intense, vivid and story-like, whereas non-REM dreams are rather prosaic by comparison — more like straightforward thoughts than elaborately imagined experiences. Usually when we think of the quintessential dream, we’re thinking of a REM dream.

The first REM period of the night usually lasts only about ten minutes, but you will have more of them as you continue to sleep: you’ll typically have between four and six episodes over the course of a night. And they will get longer as the night goes on. By the time you open your eyes the next morning you will have spent 20 to 25% of your sleep time — or around two hours — in REM. That can mean two hours of dreaming each and every night. Of course, only a fraction of these dreams will ever make it into your conscious recollection.

What’s Going On In There?

What in the world is happening inside the brain to bring all this about? REM sleep is believed to be triggered by a region of the brain called the pons, located in the brainstem, just above where the spinal column connects. The pons includes a group of neurons that become active just before a REM episode, sending out strong pulses of electrical signals known as Pontine Geniculate Occipital (PGO) waves. These waves travel up into the brain, to the lateral geniculate nucleus of the thalamus (stay with us, here) and on toward the back of the head to the occipital lobe of the cortex — both these regions happen to be heavily involved in the sense of sight. The PGO waves set off an incredibly complex pattern of activity that science is only just beginning to understand.

Once in REM, the brain is lit up like a Christmas tree with electrical stimulation, from its base to its outer cortex. The patterns you see here are very interesting: The primary visual cortex, which decodes incoming signals from the eyes, remains off-line, but other visual centers of the brain are quite active. This is not unlike what happens when you close your eyes and recall a vivid memory or visualize something in your imagination. In fact, areas that have to do with all the senses — including the sense of motion and balance — are similarly active.

This might be the key to understanding how and why we have dreams. The brain is somehow being tricked into thinking that it’s receiving sensory information from the outside world, but it’s all being generated entirely from within. The mind, it seems, has constructed its own self-contained, simulated universe.

Some of the most intense brain activity during REM can be seen in the so called limbic system, an area of the brain that has much control over our emotions, behavior, and memory functions. Three specific regions are particularly charged up here: the hippocampus, which stores away and retrieves long term memories, the amygdala, which has to do with memories of a highly emotional nature, and the anterior cingulate gyrus, which controls mental focus, concentration, and motivation. This pattern may explain why our dreams tend to play back snippets of memory and why they often have such an emotionally emphatic quality to them.

Not all areas of the brain are being stimulated though; the frontal lobe of the cerebral cortex remains relatively inactive during REM. And since this is the part of the brain that controls structured thought, such as planning, decision making and reasoning, it’s no wonder that our dreams can seem so disjointed and relentlessly weird.

Dreamland

Since your mind becomes utterly convinced that what it is experiencing is, in fact, real, it tries to interact with the dream environment — walking, talking and moving about — just as it would in a real environment. The brain actually wants to send messages to the body’s muscles, commanding them into action, in the same way it would when you’re truly awake.

Fortunately, during REM, these neurological signals are cut off in the brainstem before they have a chance to travel down the spinal cord, thus creating a temporary state of partial paralysis known as REM atonia. This is a crucial feature of REM sleep. You can imagine the dangerous situations that could result, if we were left free to physically act out our dreams.

One of the great controversies surrounding REM has always been what causes the rapid eye movements themselves. Many scientists believe that your eyes move about as you dream in direct response to what you’re seeing in your dream environment — just as you’d glance around to scan your surroundings in real life. In one remarkable experiment, sleep researchers watched as the eyes of a test subject, asleep in REM, made an incredibly rhythmic series of 26 side-to-side motions until the researchers finally woke him up. What had he been dreaming about? He was watching his friends play ping-pong!

Other scientists have come along to pour cold water on this notion. While rapid eye movements may be influenced by perceived dream images to some degree, they are said to be mostly random. In fact, the movements generally come in periodic bursts which seem to be tied into the pulses of PGO waves coming from the brainstem. Some experiments have even found that the eyes tend to move independently of each other, several degrees out of phase.

One expert has proposed that the eye movements simply function to keep the fluid inside our eyeballs stirred up during the long hours of sleep, promoting proper blood circulation within the eye and lubricating the cornea. This is certainly the most offbeat as well as the most mundane explanation for REM.

But it would seem more likely that the eye movements are really just an outer manifestation of something deeper. If we’re interested in finding the true purpose of REM, we should probably look beyond the eyes at what’s going on inside the brain.

In Search of a Purpose

So what is evolution up to here? Why has nature gone to such trouble to incorporate REM into our sleep and into the functioning of our brains? A feature this highly complex and integral certainly suggests a vital purpose.

A big clue may come from the fact that newborn babies have much more REM sleep than adults. We typically spend from 20 to 25% of our eight hours of sleep in REM — or about 2 hours a day. Newborns, by contrast, spend about 50% of their 14 to 16 hours of sleep in REM — which works out to 8 hours a day. In fact, the most REM we will ever have is before we are even born: more than 12 hours a day, by some estimates, in the last month before birth. This same pattern can be seen in animals as well, across many different species. Whatever REM does, it must be particularly important in infancy and childhood, and less so in adulthood.

This has led to the theory that REM plays an important role in the way our brains develop during infancy. The process of early brain development is nothing short of miraculous. What is particularly amazing is the extent to which it is driven by sensory stimulation. It’s the electrical signals coming in from the five senses that help carve out the neural pathways that will form the brain’s circuitry. If these signals are cut off for any reason, the corresponding areas of the brain simply fail to develop.

As we just saw, the intense brainwave patterns of REM have a way of mimicking external sensory input. So it’s possible that REM may actually accelerate brain development by complimenting the sensory stimulation so crucial to the growing brain. This could be especially important before birth, as the brain takes shape inside the womb where sensations are obviously quite limited.

This theory does, of course, leave us with a pretty big unanswered question: If the purpose of REM is early development of the brain, why do we continue to have it throughout our lives?

The answer may be that REM is so tightly integrated into brain function at the beginning of life that it just becomes a permanent part of how the brain works. Like your belly-button, REM may be nothing more than an enduring imprint of your long ago infancy.

It could also be that, in a sense, the process of brain development is never really complete. We never stop growing new brain cells (as we now know) or forging new neural pathways, so maybe REM continues to be necessary. It could also be that REM has other functions that remain essential through adulthood.

REM and Memory

There are, of course, many other theories about the purpose of REM. One is that REM helps our brains process memories and learning. There is already a substantial body of evidence suggesting that sleep, in general, plays some kind of role here. So where does REM fit in?

The idea that REM in particular supports memory consolidation is intuitively very appealing. During REM, the brain becomes intensely activated, especially in areas having to do with memory. It’s easy to imagine the brain going to work as we sleep to file away fresh memories, transferring new information from short-term into long-term memory banks, and integrating what we’ve just learned with what we already know. It’s also easy to see how our dreams could be a kind of reflection of this off-line sorting process.

The dramatically higher amounts of REM seen early in life would make sense too, given the fact that learning and new experiences are such a huge part of a child’s daily life. The young brain is kept extremely busy absorbing it all and any mechanism involved in memory would have to be working overtime.

The evidence for all this, however, is somewhat mixed. Some studies have shown that if you are selectively deprived of REM sleep after a learning experience, you’ll have more trouble recalling what you learned. Other studies have shown that learning causes temporary increases in REM sleep. Yet there are many other studies that have shown none of these effects and point more to non-REM for memory consolidation.

The biggest problem with this theory comes in the form of the many people who, for various medical reasons, have little or no REM sleep. If REM plays any role in memory, you’d expect to see some kind of memory impairment here. Yet their memories seem to work just fine without it.

Unlearning

The whole question of REM and memory was given an interesting twist about thirty years ago, by Francis Crick (co-discoverer of the double helix structure of DNA) and Graeme Mitchison when they proposed that REM facilitates what they called “reverse learning” or “unlearning.” Their theory runs like this:

Our memories are stored in vast associative networks of neuronal connections. As we accumulate new memories each day, these networks can become overloaded by irrelevant or unneeded information. To keep our memories working efficiently and accurately, some sort of “cleaning up mechanism” is needed to sort out all the mental clutter.

That, they believe, is where REM comes in. The random brainwave patterns and intense electrical stimulation generated by REM would tend to eliminate unnecessary memory traces by shaking loose weak, unstable neural connections. Meanwhile, the stronger connections which contain important memories would be left intact and ultimately reinforced. Bad or unneeded information gets deleted or “unlearned” and our memory banks are kept clean and organized. In terms of evolution this would have had the additional benefit of greatly increasing our memory capacity without requiring a physically larger brain.

This is intriguing stuff. But again, like any theory of REM and memory, it gets snagged on this question of why people who are unable to have REM show no memory impairment. If REM plays a crucial role in anything, its absence should have some kind of an observable effect.

Stimulating Your Brain

Another very different theory of REM is that it periodically activates the central nervous system during long hours of sleep to keep the brain from sinking too far into unconsciousness. In this view, the brain doesn’t like to go for long periods of time without some kind of stimulation. It needs to come up for air, so to speak.

Yet there are real advantages in being able to consolidate sleep into one large block of time. So REM gives the brain a way to keep itself stimulated without actually having to wake up, by providing a sort of “pseudo-stimulation.” This allows us to sleep for many hours at a time, yet the brain is kept tuned up and ready to go.

The fact that REM episodes get longer as the night progresses would also suggest that REM is acting to prime the brain for wakefulness as morning approaches. REM may give the brain a kind of pre-wake workout in order to speed the booting up process. When we wake up, we’re able to achieve alertness more quickly with a minimum of grogginess.

So why do infants and children have so much more REM than adults? Proponents of this theory answer that the deeper you sleep, the more REM you need in order to recover. Thus, newborns need greater amounts of REM simply because they have greater amounts of deep sleep. They also address the question of people who don’t have REM sleep. They argue that the brain is able to compensate for this by shifting sleep patterns in key ways. People who lack REM will tend to have less deep sleep and more light sleep, as well as more frequent awakenings throughout the night.

It’s been more than 60 years now since rapid eye movement was first discovered and its true function is still unknown. Like so many other matters of science, the more we learn about it, the more we open up new questions. The nature of sleep continues to defy our understanding and REM remains its biggest mystery.