NEW YORK (CBSNewYork) — Imagine being deep in a good night’s sleep only to wake up scared out of your wits and helpless.

As CBS2’s Dick Brennan explained, it’s a disorder that’s more common than you might think.

“It’s just really terrifying,” Lilly Altamirano recalled.

Instead of resting peacefully during their hours spent sleeping, some people are plunged into a world of horror – waking into a literal nightmare, unable to do anything about it.

“It was probably the scaredest I’ve ever been,” said Tim Hall.

The disorder is known as ‘Sleep Paralysis.’ People who suffer from it are awake, but unable to move.

Altamirano has had since she was a child.

“Sometimes it’s a prison. Sometimes my body’s a prison,” she said.

Hall had only one episode, but the experience is still vivid.

“All of a sudden I wake up, and I couldn’t move at all,” he said.

What makes it even worse — sufferers experience horrifying hallucinations – scary, menacing, shadow people that seem very real to a person helpless to do anything about it.

“When I see them coming toward me, I just like, I am terrified,” Altamirano said.

“It’s like being tied up in a straitjacket, and then you feel like there’s a stranger in your room,” Hall explained.

Researchers said sleep paralysis occurs when the body is moving from rapid eye movement, or 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 said anywhere from 5 to 20 percent of people experience it in their lifetime.

“Something like that happens, you don’t forget it so quick,” Rodney Ascher said.

After Ascher experienced sleep paralysis, he made a documentary that he said touched a nerve.

“A lot of people have written to me to say they’re really relieved to see something like their experience reflected back at them,” he said.

“It’s a frightening feeling, yes, definitely,” sleep expert, Dr. Divya Gupta of JFK Medical Center said.

Gupta said the disorder is not harmful and 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 had or with your toes or your fingers that will help you to snap out of it,” she said.

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.

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