TOKYO, July 5— About once a week, Jean-Christophe Terrillon wakes up and senses the presence of a threatening, evil being beside his bed. Terror ripples through him, and he tries to move or call out.

But he is paralyzed, unable to raise an arm or make a sound. His ears ring, a weight presses down on his chest, and he has to struggle for breath.

''I feel an intense pressure in my head, as if it's going to explode,'' said Mr. Terrillon, a Canadian physicist doing research in Japan. Sometimes he finds himself transported upward and looking down on his body, or else sent hurtling through a long tunnel, and these episodes are terrifying even for a scientist like him who does not believe that evil spirits go around haunting people.

Called sleep paralysis, this disorder -- the result of a disconnect between brain and body as a person is on the fringe of sleep -- is turning out to be increasingly common, affecting nearly half of all people at least once. Moreover, a growing number of scholars believe that sleep paralysis may help explain many ancient reports of attacks by witches and modern claims of abduction by space aliens.

''I think it can explain claims of witchcraft and alien abduction,'' said Kazuhiko Fukuda, a psychologist at Fukushima University in Japan and a leading expert on sleep paralysis. Research in Japan has had a headstart because sleep paralysis is well-known to most Japanese, who call it kanashibari, while it is little-known and less studied in the West.

''We have a framework for it, but in North America there's no concept for people to understand what has happened to them,'' Professor Fukuda said. ''So if Americans have the experience and if they have heard of alien abductions, then they may think, 'Aha, it's alien abduction!' ''

Sleep paralysis was once thought to be very rare. But recent studies in Canada, Japan, China and the United States have suggested that it may strike at least 40 percent or 50 percent of all people at least once, and a study in Newfoundland, Canada, found that more than 60 percent had experienced it.

There, as in Japan, people have a name for the condition and some scholars believe that people are therefore more likely to identify it when it happens to them. In Newfoundland, it is called ''old hag'' because it is associated with visions of an old witch sitting on the chest of a paralyzed sleeper, sometimes throttling the neck with her hands.

Sleep paralysis seems to have been described since ancient times, and an episode appears in ''Moby Dick'' and perhaps also in the 18th century Henry Fuseli painting, ''The Nightmare,'' which shows a goblin sitting on the stomach of a sleeping woman. What is striking is that although the symptoms of sleep paralysis are generally very similar, the images in the hallucinations and the interpretation of them seem to vary.

Europeans seem to have interpreted ancient sleep paralysis as assaults or abductions by witches taking them off for a forcible ride on a broomstick. Chinese called it ''gui ya,'' or ghost pressure, and believed that a ghost sat on and assaulted sleepers.

In the West Indies, sleep paralysis was called ''kokma'' and meant a ghost baby who jumped on the sleeper's chest and attacked the throat. In old Japan, it sometimes seems to have been interpreted as a giant devil whose foot came down on the sleeper's chest.

''People will draw on the most plausible account in their repertoire to explain their experience,'' said Al Cheyne, an associate professor of psychology at the University of Waterloo in Canada. ''Trolls or witches no longer constitute plausible interpretations of these hallucinations. The notion of aliens from outer space is more contemporary and somewhat more plausible to the modern mind. So a flight on a broomstick is replaced by a teleportation to a waiting spaceship.''

Dr. Cheyne said that in a survey he had worked on involving more than 2,000 people identified as experiencing sleep paralysis, hundreds described experiences similar to alien abduction.

''A sensed presence, vague gibberish spoken in one's ear, shadowy creatures moving about the room, a strange immobility, a crushing pressure and painful sensations in various parts of the body -- these are compatible not just with an assault by a primitive demon but also with probing by alien experimenters,'' Dr. Cheyne said. ''And the sensations of floating and flying account for the reports of levitation and transport to alien vessels.''

In recent years there has been a huge increase in the number of people who insist that they have been kidnapped by alien creatures from outer space, perhaps subjected to medical experiments and then released again. These claims have been a bit of a scientific puzzle, because they strike most people as utterly wacky and yet they are relatively widespread. One well-publicized (and widely criticized) Roper Poll published in 1992 suggested that nearly four million Americans reported experiences akin to alien abduction.

Surprisingly, one study found that these people were no more fantasy-prone than the general population and had slightly higher intelligence. Many shun publicity and show signs of feeling traumatized and humiliated.