IT WAS the wreckage of yet another TV that finally convinced one man to seek help. remembers the patient well. “When we asked what brought him in, he said, ‘Well, that’s the third TV set that I threw at an intruder who isn’t there. It’s getting damn expensive.'”
Zadra, who studies sleepwalking at the University of Montreal, is interested in why anyone would do things like this in their sleep. And it turns out that the answer is important to all of us.
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You might think that when you close your eyes and drift off, your body basically shuts down, and dreams then play out in your head. Due to the inhibition of muscle movement, or muscle atonia, that normally occurs during dreaming sleep, most of us don’t act out our dreams or have one-sided conversations. Just 1 per cent of people sleepwalk regularly. But three-quarters of us will talk in our sleep and a third of us will sleepwalk at some point. And we all occasionally shift position or mumble. Now we are learning that even the seemingly subtle twitches and murmurs we make actually have a surprisingly important impact.
Work by Zadra and others is revealing that our bodies play a far more active role in what happens during sleep than people generally think – and not just for sleepwalkers or people chucking appliances at the wall. Their findings suggest that movements in our dreaming minds, or sleeping bodies, serve a far more fundamental purpose, one that shapes how we move and talk in our waking lives.
Despite spending roughly a third of our lives in the land of nod, what exactly sleep is for, and why it is so crucial to our health, largely remains a mystery. Little by little, we are solving the problem: we now know that sleep is essential for laying down memories, clearing out the gunk that builds up in our brains during the day and even staving off cognitive decline. And there are countless ideas about the purpose of dreams – from a way to rehearse for life-threatening situations in the real world to working through thorny problems without life’s many distractions.
“Movements in our sleeping bodies shape how we move and talk in waking life”
Our sleeping movements might be more than just a glimpse into our dreams, though. It is becoming clear that a few hours’ kip makes a difference to our coordination. Research has consistently found that sleep improves performance on motor tasks, from tracing a design from a mirror image to improving reaction times or performance in sports like tennis or basketball. Such findings suggest that while we’re out, the brain is replaying our recent movements and strengthening these motor memories. So perhaps our night jiving is related to this?
Up until now, most of the evidence for the strengthening of motor memories comes from animal studies. For instance, when a rat with a chip implanted in its brain learns how to navigate a maze, we can watch in real time as the map of the labyrinth is encoded in the , an area associated with specific memory for places. When we observe the rat’s brain during rapid eye movement (REM) sleep, when most dreams occur in humans, the same neural patterns occur, as if the brain is replaying the trajectory through the maze.
To see if unusual physical movements in our sleep, known as parasomnias, could correspond with us replaying things we’ve learned, researchers have started studying some of the more animated human sleepers – from those who let out light grumbles or sit up in bed, to more extreme cases when people eat, drive or even have sex while asleep.
To try to learn more, neurologist Isabelle Arnulf and her team at the Pierre and Marie Curie University in Paris invited 19 regular sleepwalkers to spend a few nights in their lab. Before they bedded down, the sleepwalkers – and 18 people who don’t sleepwalk – learned a game that involves tapping blocks placed around their bodies as quickly as they could.
Nothing much happened during the first few nights that the participants slept in the lab. But then Arnulf and her team finally : “This patient opened her eyes and instead of running out of bed or speaking, she put her hands in the posture we had waited for… and made fictive taps in the air.” She was playing the game. In related experiments in which Arnulf’s team asked 18 sleep talkers to memorise a story, one was heard .
These fascinating findings upend some long-held ideas. Until now, we knew that motor learning took place within the brain during sleep. But these studies show for the first time that physical actions are being replayed in the dreaming world and rehearsed in the body too.
Of course, that is in sleepwalkers, who Zadra recently found are actually in . What about sleepers who are more reliably out for the count? In theory, Arnulf says, similar processes may be at play: evidence suggests that parts of the motor cortex, which controls our movements, , but it may be that the muscle atonia that keeps our bodies still prevents the final physical motion. In other words, we rehearse everything up to the point of the actual movement. But, ultimately, doesn’t that mean it’s all in our heads?
Definitely not, says at the University of Iowa, who studies sleeping movements. “Although many people think of sleep as a period of inactivity, nothing could be further from the truth,” he says. During REM sleep, in particular, our bodies move a lot, twitching or making little jerking movements, like those you might see in the whiskers of a cat or the flickering eyelids of a baby. In fact, every single skeletal muscle of the body twitches during REM sleep, from those controlling our limbs to our fingers and faces.
This twitching seems to be critical to developing knowledge of our own body. For infants, who spend about 8 hours in REM sleep each day, these movements help to draw the map that links the brain to motor neurons in the spinal cord and, in turn, onto the skeletal muscles. These are the maps we rely on throughout our waking lives: they are what underpin our ability to control our muscles, to move how and when we want to.
“Twitches are a way of probing the body,” says Blumberg. They often occur in a discrete fashion, meaning just one muscle moves at a time. In rats observed while sleeping, researchers could watch specific movements happen independently: shoulder rotation, elbow extension, wrist flexion. The same seems to be happening with us. Blumberg says to think of it like a switchboard controlling a mess of light bulbs hanging from the ceiling. You could turn the switches on one by one to figure out which switch controls which light bulb. Or even better, keep all of the other switches turned off so you can easily pick out the single bulb that flickers on. This is what we can do during REM sleep. Because the rest of our muscles are in a state of paralysis, and each twitch works on a single muscle, the signal is very clear.
Redrawing the map
For adults, twitching in our sleep is a way of recalibrating the sensorimotor system as we grow older. After all, these maps must be continually adjusted as our bodies grow and change in size and shape. And when a stroke or other kind of traumatic brain injury happens, the maps must be repaired. These new findings may explain why, in the longer term, and worsening sleep quality go hand in hand as we age, and why .
So the movements we make in our sleep help us maintain muscle control, coordination and other essential physical processes we use while awake. But that is only part of the story.
We know that our brains can drive some of our sleeping actions. Lucid dreamers, who are asleep but able to direct their dreams, find that they can control their eye movements, tap out Morse code and even perform some simple physical exercises when they’re out, for instance. And particular physical actions – such as Zadra’s patient lobbing a TV set at a perceived intruder – certainly seem to spill over from dreams.
However, our bodies may shape our dreams too. One of Blumberg’s studies shows that the sensorimotor cortex of the rat brain was five times more active during twitching in sleep than during waking movements – but most interestingly, the . The twitching seemed to be driving brain activity, not resulting from it. Jerking movements in our sleep have long been thought to reflect us acting out dreams, but this work suggests the opposite: our twitching bodies may instead have us dreaming out acts.
“Instead of acting out dreams, our bodies may be dreaming out acts”
The finding is consistent with research showing that the brain isn’t simply the puppeteer of the body, telling it what to do and when. It is a two-way street, and we often incorporate outside sounds or sensations into our dreams. Perhaps the most famous example of this is from the late 1800s, when the French physician Alfred Maury dreamed he was being guillotined after his headboard fell onto his neck. In more recent work at the , researchers found that inflating a pressure cuff on the leg of one participant was associated with a dream about their .
Determining when these actions originate in the brain or body isn’t just an academic matter: it can make a real difference to how we treat sleep disorders when people act out in ways that can cause harm. For sleepwalkers, most actions are nothing to worry about, even if it is bewildering to wake up munching a teabag sandwich or repainting the front door (see “Sleep troubles”). They can be far more unsettling though, as in cases of so-called sexsomnia. One man had sex with his wife nightly for an astonishing 13 years without remembering – and didn’t believe her when she said he’d forgotten until a doctor confirmed it. Occasionally, though, things are more dangerous, such as when US comedian jumped out of a window while sleeping. They can also be early signs of serious illness or include outbursts of violence.
A condition often linked to violent outbursts in sleep is REM sleep behaviour disorder, in which normal, paralysing muscle atonia and the jerky twitches of REM sleep are both disrupted. It is marked by actions such as kicking and flailing, which often coincide with dreams about fighting or fleeing from danger. It is possible that these dream movements are driven by exaggerated muscle twitches emanating from the body, but their sometimes complex nature suggests this isn’t always the case: people with the condition have been recorded breaking into song or reciting memorised political speeches.
It can be that an underlying sleep disorder, such as sleep apnoea, triggers sleepwalking episodes. But many of the most successful treatments, including cognitive behavioural therapy, target waking behaviours that can up the chances of night-time shenanigans. There is evidence that poor sleep habits, stress, alcohol, antidepressant drugs, sleeping pills and other medications can increase the likelihood of sleepwalking or talking by interfering with the transition between sleep stages or decreasing sleep depth.
For people like Zadra’s TV-throwing patient, for instance, cutting back on alcohol, getting into a better sleep routine or avoiding certain medications can be enough to interrupt the harmful cycle.
Even for those of us who seem to slumber soundly, these insights reveal that our sleeping bodies are far more active than we knew. As we begin to build a picture of what sleep is for, it seems our dreams play a crucial role in shaping our ability to move through the waking world. Yet another reason not to skimp on a good night’s sleep.
Sleep troubles
We all move a bit in our sleep: muscles in the body twitch, our eyes flit back and forth during dreams and facial expressions and short vocalisations are common (see main feature). So when do these behaviours become a cause for concern? Behaviours like sleep talking or sleepwalking typically begin at younger ages, are relatively benign and become less common as we get older. The odd episode certainly isn’t anything to fret about, but if it begins to undermine your quality of life or becomes violent then you should seek medical advice.
For 1 in every 100 people, routinely acting out in sleep may be a sign of REM sleep behaviour disorder (RBD), in which our normal inhibition of physical movements during sleep fails. The condition features violent grabbing, punching, kicking and flailing that happens from 20 to more than 100 times a year, usually during dreams involving aggression, fleeing or attack. It affects 1 in 12 men over 60.
Some , often Parkinson’s, within 14 years of their symptoms first appearing.
Strangely, although people with Parkinson’s have impairments in movement and voice control while awake, recordings of movements and talking in their sleep show that they regain their normal voice and move without tremors.
Some researchers think this is because the sleeping actions initiate in the brain stem, whereas it is problems in the motor cortex that may be responsible for tremors and other impairments when people are awake. But as Parkinson’s is caused by a lack of the neurotransmitter dopamine, the increased dopamine levels seen during REM sleep might also explain the restored coordination.
“We dream of reactivating this elevated motor control in waking life,” says neurologist Isabelle Arnulf at the Pierre and Marie Curie University in Paris. There is also hope that, given the link between the two, treating RBD could reduce the risk of neurodegenerative disease.
This article appeared in print under the headline “Night exercises”
