HOUSTON -

Imagine waking up to find yourself completely paralyzed, aware of your surroundings but unable to even open your eyes. It's a real, often terrifying condition called sleep paralysis.

What started as 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, like I was paralyzed. "

Sepulveda experienced sleep paralysis, an episode in which a person is consciously awake but can't move. Often, as in Sepulveda's case, it includes what many describe as a paranormal experience.

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

There are supernatural theories for sleep paralysis that date back hundreds of years.

"Some cultures believe it's demonic," said Johnny Castillo, with Supernatural Research Society. "They call it incubuses -- succubuses."

Other theories include everything from ghostly visits to alien abductions. But there's also a scientific explanation.

"What causes it is rapid eye movement sleep, or an abnormal part of the sleep, which is when REM gets into your waking up or falling asleep," said Dr. Angela Valez, a University of Texas Medicine sleep specialist.

In other words, the part of brain that paralyzes us to keep us from acting out 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.

"I haven't had that happen anymore, but who knows? It might happen again," Sepulveda said.

Doctors say isolated episodes can happen to just about anyone due to stress or a lack of sleep.

Sleep paralysis is often also associated with panic attacks and other anxiety disorders. If you have frequent episodes, you should see a doctor to help treat the condition.