Science Writer Sam Kean has had real-life experiences worthy of a horror film. During one such recent episode, he woke up from a night of sleep and found that he was unable to move, completely paralyzed, but fully awake.

Kean isn’t the only one who’s had such an experience. Others have had similar episodes — waking from sleep to what they described as a “demon sitting on their chest” or an alien abduction. One woman in this state was even thought to be dead and was taken to a morgue before recovering the ability to move her limbs.

The episode, as horrific as it sounds, has a very real scientific explanation.

“It's a condition called sleep paralysis and a lot of people actually do experience it maybe a couple times in their life. Some of us, unfortunately, experience it on a more regular basis and I guess I'm one of the unlucky people,” Kean says, “What happens is your mind wakes up, but your body doesn't wake up. So you're sitting there as if you were a statue or something like that. You just cannot move anything in your body no matter how hard you try. It's really kind of an awful condition to be in.”

Kean, inspired by his experience and other research, has written a book called “The Tale of the Dueling Neurosurgeons: The History of the Human Brain as Revealed by True Stories of Trauma, Madness, and Recovery.”

“What I was trying to do with the book is kind of march around from place to place and show you what happens when each part of the brain shuts down. Unfortunately a lot of these things end up with these really bizarre consequences,” Kean says.

In the case of sleep paralysis, Kean says, there may be an explanation for the bizarre experiences people report of being abducted by aliens, or having other strange experiences.

“Unfortunately for some people it goes even a step further because their brains don't shut off the dreams. So they're awake on some level, their bodies can't move, and they have these fantastical things going on in their mind. So they interpret them as say a witch or something squatting on their chest or aliens might have them strapped down for some reason to examine them. So that's how their brain makes sense of the experience.”

Another experience Kean describes is that of Walking Dead Syndrome or Walking Corpse Syndrome where “people are convinced that they’re dead. They are sitting there talking to their doctors, they’re moving, they're breathing, but they are convinced that they're dead to the point where some people claim they can smell their own flesh rotting. A few people have apparently tried to cremate themselves. They are absolutely convinced that they are no longer living.”

Scientists aren’t exactly sure what causes Walking Corpse Syndrome, or Cotard Delusion, but some theorize that it could have to do with a sort of “emotional glow” that people get when they see themselves in the mirror, an ability that goes beyond recognizing emotional features, to realizing, at an emotional level, that the person in the mirror is actually themselves. “Maybe these people aren't feeling that emotional glow for themselves and they're interpreting this for whatever reason as the fact that they're dead,” Kean says.

Another phenomenon Kean explores is that of the “Alien Hand” in which a person is unable to control their own hand.

“Your own hand, without your permission, will jump out and grab a pot of boiling water on the stove, or grab the steering wheel. There are cases of one hand trying to button up a shirt and the other hand coming along right behind it and unbuttoning the shirt. So the hands are either grabbing on to things you don't want to, or they’re kind of in active opposition to each other,” Kean explains.

The syndrome, Kean says, is perhaps the result of damage to people’s sensory equipment, meaning their brains aren’t getting the normal feedback to tell them when they’ve made a certain movement.

Another theory about alien hand syndrome is that it involves “damage to the frontal part of the brain which is kind of the executive that tamps down on impulses in the other part of the brain. And when that frontal part gets damaged, your impulse to — say, grab a knife, or grab a pot of boiling water — it can't suppress those impulses as well and so your hand just kind of does what it wants,” Kean says.

The good news about these horror-story symptoms?

“They are very rare. They are well established, we know that these things exist, but they are pretty rare conditions. And usually what has to happen is multiple parts of the brain have to go down at once. It's not like you get a stroke in some little tiny area and suddenly this happens. There usually has to be pretty specific and even extensive damage for these things to take place.”

This article is based on an interview that aired on PRI's Science Friday with Ira Flatow.