麻豆传媒

Read more: 鈥�The night: The nocturnal journey of body and mind鈥�

THE camera is trained on a man in the grip of an agitated sleep. He has thrown off the covers. Now he hoists himself into a kneeling position. His mouth slack with sleep, he starts punching desperately and repeatedly. Suddenly he flings his arms over his head, gripping what seems to be an invisible sword. He plunges it into the bed, over and over, his fists making deep impressions in the mattress next to him, where another sleeper might be.

Like many sleep-clinic recordings, this one is 鈥渞eally creepy鈥�, says John Peever, a neurologist who studies sleep disorders at the University of Toronto in Canada. And bizarre, too: this man鈥檚 brain is asleep, and yet his body moves with vigour and violence. People come to these clinics to seek help. Perhaps they have eaten things asleep that they鈥檇 never go near during the day 鈥� some ice maybe, or dog food, or dishwashing detergent. They might have had sex without knowing it. They might have dreamed of trying to save a child or partner, only to wake and find they鈥檙e hurting them instead.

These are parasomnias, sleep disorders that blur the supposedly set lines between wakefulness and sleeping. As we start to unravel the mechanisms that might be at work, we are finding that the boundaries between states of consciousness are more porous than we expected.

We exist in essentially three states: awake, in deep sleep, and in dreaming sleep. Awake, we move consciously and responsively through the outside world. In deep sleep, we move, twitch and turn but not on purpose, largely unaware of the outside world. Dreaming, we do not and cannot move, and the only world is the one inside our brain. For a long time, sleep researchers assumed that we exist in only one of these states at a time.

That assumption was reinforced by the distinctive way our brains and bodies behave in the different states. The states are 鈥渇undamentally different creatures鈥�, says , a neurologist at the University of Minnesota, Minneapolis, who directs numerous sleep medicine programmes. In rapid-eye-movement (REM) sleep, the brain is active and intensely dreaming. Your body is effectively 鈥渙ff鈥� and no longer able to control its temperature; your skeletal muscles are frozen to keep you from acting out your dreams. Non-REM, or NREM, sleep is 鈥渁s different from REM as it is from wakefulness鈥�, says Howell. Here, the brain gets less active, limiting itself to light, prosaic dreams. Your muscles are slack, but you can move.

Peever says neurologists more-or-less understand how we stay in each state. What is less clear is how the states change from one to another. Any good engineer would have the states controlled by a three-way master switch: if REM is on, then NREM and wake are off. But if such a switch exists, parasomnias make it look like it has a lot of play in it. In fact, parasomnias made researchers wonder how separate the states really are. In many of these disorders, sleep seems to get mixed up with aspects of wakefulness.

That is most apparent in NREM parasomnias, which are the most common. The parts of the brain in charge of moving, talking and navigating have slipped into wakefulness. But all the rest, including the parts of the brain that make us aware of movements and can monitor them, is still offline. The result is an unconscious creature that can walk, eat, have sex and scream. The names of the disorders are just descriptions: sleepwalking, sleep eating, sexsomnia and sleep terrors. No one remembers anything the next day, but the evidence gives them away: one woman awoke, feeling unaccountably ill, to discover a half-eaten, buttered cigarette.

鈥淥ne woman awoke feeling ill to discover a half-eaten, buttered cigarette鈥�

There鈥檚 more than one form of consciousness (Image: Harry Gruyaert/Magnum Photo)

The REM parasomnias are harder to explain. One, called sleep paralysis, is so terrifying that it has been blamed for reports of . While your muscles are still immobilised, the dreaming part of your brain seems to wake up 鈥� leaving you conscious but not fully awake. Because REM sleep dreams are 鈥渧ivid, bizarre, loosely associated, and very emotionally charged鈥�, Howell says, the result is a feeling of a malevolent presence in the room 鈥� often evil, certainly frightening, and wholly inescapable.

The other REM parasomnia 鈥� REM sleep behaviour disorder (RBD) 鈥� inflicts the same vivid and disturbing dreams, but turns off the paralysis that keeps you from acting on those dreams. Now your body does what the dream directs. When the dream is violent, real people have been badly hurt and even killed.

Researchers are starting to zero in on some of the physical mechanisms behind the REM sleep disorders. Peever and his colleagues have found one possible explanation for why we remain paralysed despite waking. During normal REM sleep, two chemical messengers called GABA and glycine are released in the brain to stop us moving. They are supposed to when REM sleep ends, but during sleep paralysis they seem to persist, even as other parts of the brain are waking up.

RBD has a different cause and can suggest something more serious going on in your brain. Unlike the other parasomnias, it is probably a result of a lesion somewhere in the complex neural pathway that paralyses the body during REM sleep. Often, if a person has RBD, .

RBD aside, the idea that wide or leaky boundaries between states of consciousness account for almost all parasomnias has gained traction in recent years. 鈥淲ake and sleep are not mutually exclusive,鈥� says Mark Mahowald, who directs the Minnesota Regional Sleep Disorders Center in Minneapolis. 鈥淭hey鈥檙e graded.鈥�

Cross-boundary intrusions could also account for other lapses of consciousness. Deprived of sleep long enough, for example, waking people hallucinate. 鈥淭he pressure to dream is so great, you go into REM sleep when you鈥檙e awake,鈥� Mahowald says.

Indeed, with people switching states every night of their lives, perhaps the real mystery is why parasomnias don鈥檛 happen more often.

This article appeared in print under the headline 鈥淓yes wide shut鈥�