By Stephanie Stahl

PHILADELPHIA (CBS) –– With Halloween coming up, fright nights will be popping up all over, but those scary nightmare scenarios are real for some people who suffer with an unusual sleep disorder. Many people have occasional nightmares, but some have the disorder where they’re awake, living through some frightening situation and they’re unable to move.

Scared instead of resting peacefully, for some, the hours spent sleeping they are instead, plunged into a world of horror unable to do anything about it.

It’s a disorder called sleep paralysis. People who suffer from it are awake, but unable to move. “It’s just really terrifying,” said Lilly Altamirano who’s had it since she was a child. “Sometime’s it’s a prison. My whole body’s a prison,” she explained.

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Tim Hall had only one episode, but the experience is still vivid. “It was probably the scariest I’ve ever been, all of a sudden I wake up and I couldn’t move at all,” Tim said.

They experience horrifying hallucinations, scary, menacing shadow people that seem very real and they are helpless to do anything about it. “When I see them come toward me I just like, I’m terrified,” Lily said. Tim adds “It’s like being tied up in a straight jacket and then you feel like there’s a stranger in your room.”

Researchers say this occurs when the body is moving from rapid eye movement of REM sleep to wakefulness. For reasons not understood, the transition is not completed. The person is awake, but the body’s ability to move is not yet activated.

They say anywhere from five to 20 percent of people experience it.

“It is a frightening feeling, yes, definitely,” said sleep expert Dr. Divya Gupta. She says the disorder is not harmful and she says there are ways to get through it with less fear. “It will usually pass if you just relax, or try to initiate some movement with your hand, or with your toes, or your fingers that will help you to snap out of it.”

Sleep paralysis often occurs during periods of sleep deprivation and stress. Establishing a regular sleep schedule helps as does sleeping on your side instead of your back.

Stephanie Stahl