Have you ever woken up but been unable to move; felt a powerful pressure holding you down, gripping you tight? Haruki Murakami has, and he describes it like this: “I was having a repulsive dream — a dark, slimy dream. … After I awoke, my breath came in painful gasps for a time. My arms and legs felt paralyzed. I lay there immobilized, listening to my own labored breathing, as if I were stretched out full-length on the floor of a huge cavern.”

In this excerpt from the short story “Sleep” from his “The Elephant Vanishes” collection, Murakami is describing an episode of sleep paralysis, which is better known in Japan as kanashibari (literally, “bound in metal”).

In other cultures, the experience has been attributed to a ghost (China and Korea), a demon feeding on the living (Fiji) and, in the southern United States, to a witch.

People all over the world experience kanashibari, but with accounts going back at least as far as the kaidan (ghost stories) of the Edo Period (1603-1867), it has a particularly Japanese flavor.

In Japan, too, the phenomenon seems to disproportionately affect young people, while homegrown horror films often play on a psychological fear of ghosts manifesting in the real world. It’s no surprise that these movies depict young people as being disproportionately affected by these apparitions.

Fiction that may be, but in fact a nationwide survey of junior and senior high school students in Japan conducted in 2011 by Yoshitaka Kaneita of Nihon University School of Medicine in Tokyo, and colleagues, found that of the 90,081 questionnaires analyzed, 35.2 percent of respondents reported having nightmares, and 8.3 percent experienced kanashibari — compared with 6.2 percent of the general population in the United States who separate studies have found to report instances of kanashibari.

The results of the Japanese survey — as reported in the journal Sleep Medicine (DOI reference: 10.1016/j.sleep.2010.04.015) — show that Kaneita and his team found a number of factors that seem to increase the chance of having a nightmare or experiencing an episode of kanashibari.

For nightmares, they are: drinking alcohol, having trouble going to sleep, poor mental health — or simply being female, though there is little to be done about this factor.

For kanashibari, males were found to be more susceptible than females, with their odds of affliction shortening more due to taking a long daytime nap, having an early or late bedtime — and again, having difficulty going to sleep and/or poor mental health.

Kaneita’s team concluded that regular sleep habits are important to help prevent nightmares and episodes of sleep paralysis. However, Japanese high school students are often cited as being overloaded with work to the point that they have to get by on as little as four hours’ sleep a night. Indeed, in a separate survey of 3,478 Japanese high school students aged 16 to 18 (equivalent to 10th- through 12th-graders), researchers found that on average they slept for 6.3 hours, going to bed at 00:03 and rising at 06:33.

As the evidence suggests that teenagers in both Japan and South Korea are more sleep-deprived than those in Western countries and China, Kaneita and his coauthors recommend that health education about regular sleep habits should be promoted among Japanese adolescents.

Kanashibari — attributed to supernatural causes for centuries — may even shed light on the surprising mysteries of that most ubiquitous, beguiling and enjoyable of experiences: sleep. By monitoring volunteers in sleep labs, scientists have found that episodes of sleep paralysis occur when rapid-eye-movement (REM) stages of sleep overlap with waking stages.

Nonetheless, for something so intrinsic to the human condition, sleep remains poorly understood. There are many explanations, and probably many are correct. Sleep has numerous functions. It helps regulate emotions (we’re invariably in a better mood after a good sleep); it helps us recharge and conserve energy; it helps the brain to process memory. Sleep also helps the automatic functioning of the body — the heart rate, breathing and hormone production.

What seems to happen in kanashibari is that the person starts to wake up while REM sleep is still continuing. This leads to a situation where you are aware but “trapped” in a frozen body — because during REM sleep the muscles are paralyzed.

We’ve all probably experienced moments of this intriguing and (to me) pleasurable feeling of the consciousness “floating,” unmoored to the body. But it’s only ever been a few seconds for me. If the feeling lasted for minutes or even hours, as it does for some unfortunate people, I can see how if could generate panic.

It turns out that the brain becomes hyper-vigilant during these episodes, and can hallucinate a presence — a supernatural being, it may seem — holding them down. For some people it’s an evil cat, or a witch, or the bedclothes appear to become the twisted limbs of a dead body pressing on top of them.

But let’s not end on such a horrific note. Consider, for instance, dolphins and seals: When they sleep, half the brain is dormant while the other half stays alert. What on Earth does that feel like?

Perhaps dolphins have nightmares — being trapped in a cove by Japanese drive fishermen armed with spears, for example — but whatever their experience, it doesn’t last long: The sleep of marine mammals tends to occur in short bursts. Sweet dreams to all in this new year of 2013!

Rowan Hooper (@rowhoop on Twitter) is the News Editor of New Scientist magazine. The second volume of Natural Selections columns translated into Japanese is published by Shinchosha at ¥1,500. The title is “Hito wa Ima mo Shinka Shiteru (The Evolving Human).”

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