"This went on for several years," says Clete Kushida, MD, PhD, an associate professor of psychiatry and behavioral science at Stanford University. "It was very difficult. She was exhausted." It turns out the woman had a sleep disorder called sleep paralysis -- when a person is asleep, but immobilized. Like many who have sleep paralysis, she was also having "hypnagogic hallucinations " that she was being attacked. "It's not a serious condition," Kushida says. "But it can be very disturbing."

The woman was in her late 50s. Every night she would fall asleep and then dream that she was unable to move, but that her husband was coming into her room and trying to attack her. Helpless, she could neither move nor cry out.

Sleep Paralysis Causes

Just why or how it happens isn't clear. Researchers believe sleep paralysis is caused by a disturbed rapid eye movement cycle because it mostly happens as people are falling into or coming out of REM sleep. During that stage, their brains normally paralyze their muscles anyway -- so they don't act out their dreams. But during sleep paralysis, the sleeper is awake, or half awake, and so is aware she cannot move.

Studies show that between 25% and 50% of Americans have had sleep paralysis at least once. Many people who have it also have narcolepsy, in which they fall asleep uncontrollably. Sleep experts believe sleep paralysis might be partly genetic.

Other causes include stress and disrupted sleep schedules (think jet lag or pulling an all-nighter). Several studies have also found links between social anxiety or panic disorder and sleep paralysis.

Clearly, an episode of sleep paralysis can be scary, which has led to some unorthodox theories. Research shows that people in countries as diverse as China, East Africa, Mexico, Newfoundland, and the United States have long believed that paralysis is caused by demons, witches, or other supernatural creatures sitting on their chests and sometimes trying to have sex with them.

Often the experience is accompanied by noises (like loud buzzing), sensations of being dragged out of bed or flying, and difficulty breathing. In fact, some researchers believe sleep paralysis is what's really going on with stories of alien abductions.