''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.