Credit: F. Scott Schafer/ Corbis
As a university STUDENT in 1964, David J. Hufford met the dreaded Night Crusher. Exhausted from a bout of glandular fever and studying for finals, Hufford retreated one December day to his rented university room and fell into a deep sleep.
An hour later, he awoke with a start to the sound of the bedroom door creaking open - the same door he had locked and bolted before going to bed. Hufford then heard footsteps moving toward his bed and felt an evil presence. Terror gripped the young man, who couldn't move a muscle, his eyes plastered open in fright.
Without warning, the malevolent entity, whatever it was, jumped onto Hufford's chest. An oppressive weight compressed his rib cage. Breathing became difficult and Hufford felt a pair of hands encircle his neck and start to squeeze. "I thought I was going to die," he says.
At that point, the lock on Hufford's muscles gave way. He jerked up and sprinted several blocks to take shelter in the student union building. "It was very puzzling," he recalls with a strained chuckle, "but I told nobody about what had happened."
Hufford's perspective on his strange encounter was transformed in 1971. He was at that time a young anthropologist studying folklore in Newfoundland, Canada. Some of the region's inhabitants told him about their eerily similar night-time encounters and called the threatening entity the 'Old Hag'. Most cases unfold as follows: a person wakes up paralysed and perceives an evil presence, then a hag or witch climbs on top of the petrified victim, creating a crushing sensation on his or her chest.
It took Hufford another year to establish that what he and the others had experienced corresponds to the event, lasting seconds or minutes, that sleep researchers call sleep paralysis. Although widely acknowledged among traditional cultures, sleep paralysis is one of the most prevalent, yet least recognised, mental phenomena for people in industrialised societies, Hufford says.
Nearly 40 years after Hufford's discovery, sleep paralysis has begun to attract intensive scientific attention. Several years ago, a special issue of the journal Transcultural Psychiatry included a series of papers on the condition's widespread prevalence, regional varieties and mental health implications.
Sleep paralysis differs from nocturnal panic, in which a person awakens in terror with no memory of a dream. Neither does sleep paralysis resemble a night terror, in which a person suddenly emerges from slumber in apparent fear, flailing and shouting, but then falls back to sleep and doesn't recall the incident in the morning.
Curiously, although the word nightmare originally described sleep paralysis, it now refers to a fearful or disturbing dream, says Hufford, now at the Pennsylvania State Medical Centre in the U.S. town of Hershey.
Several hundred years ago, the English referred to night-time sensations of chest pressure from witches or other supernatural beings as the "mare", from the Anglo-Saxon 'merran', meaning to crush. The term eventually morphed into nightmare - the crusher who comes in the night.
Sleep paralysis embodies a universal, biologically based explanation for pervasive beliefs in spirits and supernatural beings, Hufford argues. The experience thrusts mentally healthy people into a bizarre, alternative world they find difficult to chalk up to a temporary brain glitch.
Hufford doesn't believe that an invisible force attacked him in his college room or during several sleep paralysis episodes that have occurred since then, but he sees the appeal of such an interpretation.
"We need to deeply question two centuries of assumptions about the non-empirical and non-rational nature of spirit belief," he says.
Since the mid 1990s, psychologist J. Allan Cheyne of the University of Waterloo in Canada has collected more than 28,000 tales of sleep paralysis. According to one of the chroniclers, "The first time I experienced this, I saw a shadow of a moving figure, arms outstretched, and I was absolutely sure it was supernatural and evil."
Another person recalled awakening "to find a half-snake, half-human thing shouting gibberish in my ear." Yet another person reported periodically waking with a start just after falling asleep, sensing an ominous presence nearby. The tale continues: "Then, something comes over me and smothers me, as if with a pillow. I fight but I can't move. I try to scream. I wake up gasping for air."
Many who experience sleep paralysis also report sensations of floating, flying, falling or leaving their body.
The condition's primary emotion - terror - sometimes yields to feelings of excitement, exhilaration, rapture or ecstasy. "A small number of people, while acknowledging fear during initial episodes of sleep paralysis, come to enjoy the experience," Cheyne says.
Cheyne runs a web site where visitors fill out surveys about their experiences during sleep paralysis (watarts.uwaterloo.ca/~acheyne/S_P.html). Several thousand individuals also provide online updates about recurring episodes.
It doesn't surprise Cheyne that those who contact him seem to be average, emotionally stable folk. In surveys he has conducted with large numbers of university students and other volunteers, about 30% report having experienced at least one incident of sleep paralysis.
Roughly one in 50 people cite repeated episodes, often one or more each week. Cheyne regards the sights, sounds and other sensations of sleep paralysis as hallucinations that share a biological kinship with dreaming.
Cheyne notes work by Japanese researcher Kazuhiko Fukuda of Fukushima University. Fukuda enlisted volunteers who had experienced many incidents of sleep paralysis. In a sleep laboratory, the Japanese team monitored the volunteers, rousing them at various times during the night to trigger the phenomenon. The researchers found that during sleep paralysis, the brain, suddenly awake, nonetheless displays electrical responses typical of sleep characterised by rapid eye movement (REM).
Two brain systems contribute to sleep paralysis, Cheyne proposes. The most prominent one consists of inner-brain structures that monitor one's surroundings for threats and will launch a response to perceived dangers.
REM-based activation of this system, in the absence of any real threat, triggers a sense of an ominous entity lurking nearby. Other neural areas that contribute to REM-dream imagery could draw on personal and cultural knowledge to flesh out the evil presence.
A second brain system, which includes sensory and motor parts of the brain's outer layer, distinguishes one's own body and self from those of other creatures. When REM activity prods this system, a person experiences sensations of floating, flying, falling, leaving one's body and other types of movement, Cheyne says.
Hufford, however, regards the intrusion of REM activity into waking moments as an inadequate explaination for sleep paralysis. Dream content during REM sleep varies greatly from one person to another but descriptions of sleep paralysis are remarkably consistent.
"I don't have a good explanation for these experiences," he says.
Psychiatrist Devon E. Hinton has heard his share of terrifying stories. While sitting in Hinton's office in Lowell, Massachusetts, a 48-year-old Cambodian woman recounted two such tales from her own life. The first detailed nearly weekly nocturnal events of a type known among her fellow Cambodians as "the ghost pushes you down".
At these times, the woman said, she awakens from sleep unable to move. Three ghastly demons stalk into her room, each covered in fur and displaying long fangs. One of the creatures then leans close to her head; the second holds down her legs; and the third pins down her arms. She told Hinton when these terrors befall her, she knows the demons want to scare her to death and she feels they might succeed.
Her second tale was even more dreadful. She told Hinton that the ghost terrors usually trigger a flashback to an actual incident that occurred more than 20 years ago. Before reaching the United States, she survived the genocidal reign of Cambodian dictator Pol Pot, who directed the slaughter of roughly two million Cambodians.
As a youth, the woman witnessed soldiers escorting three blindfolded persons, whom she recognised as friends from her village, into a nearby clump of trees. Soon after, she heard the sickening sounds of her friends being clubbed to death.
In his therapy session, Hinton, who speaks the woman's Khmer language, asked her to establish a connection between the two sets of stories. She told him that the three demons are the spirits of her three executed friends, who return to haunt her so she won't forget them. She also related her worries that a sorcerer would make the spirits enter her body, causing insanity, or instruct the spirits to place objects inside her, causing anxiety and physical illness.
Each ensuing episode of sleep paralysis over the years has intensified the woman's flashbacks, sleep difficulties and other symptoms of what psychiatrists call post-traumatic stress disorder (PTSD). Hinton says that many Cambodian refugees relive past horrors through sleep paralysis. He notes that few people discuss these incidents with their physicians. "Unless you specifically ask about sleep paralysis, you don't know that a patient has it," Hinton says.
So, Hinton surveyed people at his outpatient clinic in Lowell, which has the second-largest Cambodian population in the country. Of the 100 consecutive Cambodian refugees whom Hinton saw as patients at the clinic in 2003, 42 reported currently experiencing at least one episode of sleep paralysis each year.
Most reported seeing an approaching demon or other entity during the episode that created pressure on their chests and typically triggered panic attacks. Among the refugees questioned, 45 had been diagnosed with PTSD. Of those, 35 reported being afflicted by sleep paralysis, usually with at least one episode a month.
The Cambodians told Hinton that sleep paralysis permits people who suffer unjust deaths to haunt the living and creates "bad luck". These cultural ideas foster panic attacks, he asserts.
Panic attacks, PTSD and other mental disorders may indirectly promote sleep paralysis by disrupting the sleep cycle and yanking people out of REM sleep during the night, he adds. Other factors that disturb sleep, such as jet lag and shift work, have also been linked to sleep paralysis.
Psychological treatment that delves into the personal meaning of bouts of sleep paralysis reassures sufferers that these encounters aren't signs of physical illness or supernatural visits, Hinton says.
Evidence from Shanghai also supports a connection between sleep paralysis, PTSD and panic attacks. Albert S. Yeung of Massachusetts General Hospital in Boston and his team interviewed 150 psychiatric outpatients in Shanghai. About one quarter of the patients had experienced sleep paralysis at least once, and more than half of those with PTSD or panic attacks described incidents of sleep paralysis, according to Yeung.
However, unlike the Cambodian immigrants whom Hinton studied, nearly all of Yeung's Chinese study participants regarded the incidents as innocuous in retrospect. Most had experienced feelings of dread but didn't encounter supernatural creatures.
For African Americans who experience panic attacks, sleep paralysis is also especially common, according to community surveys conducted by psychologist Cheryl M. Paradis of Marymount Manhattan College in New York. Although 25% of the African-American participants reported having experienced sleep paralysis, nearly 60% of those who had panic attacks said they regularly experienced sleep paralysis. In contrast, sleep paralysis turned up among only 7% of those of Anglo-Saxon ethnicity who have panic attacks, Paradis says.
High stress levels in African Americans, at least partly the result of poverty and racism, contribute to anxiety, sleep problems and sleep paralysis, she suggests.
Sexual abuse may also make a person susceptible to sleep paralysis. Harvard University psychologists Richard J. McNally and Susan A. Clancy have found that of the adults who were sexually abused during childhood, nearly 50% describe at least one past episode of sleep paralysis. In their study, only 13% of participants who hadn't been sexually abused reported sleep paralysis.
Long-standing sleep disturbances in those who were sexually abused may foster the phenomenon, McNally suggests.
There is a kinship between waking nightmares starring Night Crushers and reports of alien abductions, McNally and Clancy found. For more than a decade, they have been studying people who claim to have been abducted by aliens from outer space. McNally and Clancy are convinced that these claims derive from sleep-paralysis hallucinations.
Accounts of alien abductions typically begin with someone waking in the night lying face up, McNally says. The person can't move but senses electric vibrations. A feeling of terror makes breathing difficult. Alien beings advance to the foot of the bed or climb on top of the person, who then experiences a sense of floating or of being transported to an alien craft.
Days or weeks later, in response to a therapist's suggestions, the abductee may generate details of being sexually probed or otherwise assaulted by the aliens, McNally argues.
Claims of abductions by space aliens trigger much controversy, media attention and ridicule. The late Harvard psychiatrist John Mack fuelled the hubbub by defending the accounts as descriptions of actual encounters with visitors from other planets.
There's another, far more likely explanation for the reported experiences of the 'abductees', says McNally. The traumatic encounters that a person seems to experience during sleep paralysis feel as vividly real as anything that happens during the day, he notes.
Despite their fantastic claims, these people are mentally healthy, says McNally. "Sleep paralysis is an entirely natural phenomenon," he remarks. "In isolated cases, it's no more pathological than a case of the hiccups."
McNally and Clancy linked the claims of 10 alien abductees to episodes of sleep paralysis. Memories of the incidents sparked heart-rate increases and other physiological stress reactions that exceeded even those previously reported by Vietnam veterans with PTSD as they recalled combat events.
Even the most rational people who experience sleep paralysis find it difficult to dismiss their night-time ordeals as unreal, Hufford notes.
He has interviewed many U.S. medical students who, even after hearing about REM sleep and the brain's threat-detection system, insist that their frightening meetings with the Night Crusher were real. Until sharing their stories with Hufford, most of the students had never told them to anyone.
"I suspect that millions of people in the United States are walking around never having told anybody about having these terrifying experiences," Hufford says.
That's unlikely to change anytime soon, he adds. Scientists and physicians treat reports of mingling with supernatural creatures as evidence of mental imbalance. And mainstream religions condemn connections with ghosts or evil presences.
But the world of sleep works according to its own rules. Whether shunned or embraced, Hufford says, the Night Crusher returns with frightening regularity.
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Bruce Bower is a science writer based in Washington DC.
First published in Science News.
