What started as a normal night's sleep ended with a waking nightmare for David Sepulveda.

"I felt a hand on me," Sepulveda said. "I tried to get up and see what it was but I couldn't move. It was like I was paralyzed."

Sepulveda experienced sleep paralysis- an episode in which a person is consciously awake but can't move.

"It was like I was given some sort of paralyzing drug that doesn't allow your muscles to move. I couldn't open my eyes or anything," said Sepulveda.

Often, as in Sepulveda's case, it includes a paranormal experience.

"It felt like if someone was in the room and I was very cold. That's the other thing, there was a very cold feeling," Sepulveda said.

According to Johnny Castillo, with Supernatural Research Society, there are paranormal theories for sleep paralysis that date back hundreds of years.

"Some cultures believe it's demonic. They call it incubuses- succubuses," said Castillo.

Other theories include ghostly visits or alien abduction. Of course there's also a scientific explanation, according to Angela Velez, MD- UT Medicine sleep specialist at UT Health Science Center.

"What causes it is REM sleep or an abnormal part of the sleep, which is REM, gets into your waking up or falling asleep period," said Velez.

In other words, the part of brain that paralyzes us at night, as protection for acting out our dreams, fails to switch into waking mode.

Episodes can last a minute or two. Often it's a symptom of a sleep disorder called narcolepsy, in which hallucinations are common.

The hallucinations could explain the presence Sepulveda felt in the room but he's not sure it explains why the room got cold.

"Of course I don't believe in spiritual stuff like that. Of course, I joke around about it but it's not like I believe in it," said Sepulveda. "However, now I don't know. It's made me a possible believer now."

According to Velez, while sleep paralysis is often a symptom of narcolepsy, isolated episodes can happen to just about anyone due to stress or a lack of sleep.

Anyone who experiences these episodes frequently should see a doctor as it is treatable.

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