Check out this article. Here's an excerpt:

As Clancy and McNally interviewed the abductees, they started to find some common threads. Many of them, for instance, described the terrifying experience of waking up and being unable to move, certain that an intruder was lurking in their room.

To the Harvard psychologists, it was obvious that their subjects had suffered an episode of sleep paralysis — a state of limbo between sleep and being awake, sometimes punctuated by hallucinations.
[/b]
Just thought you all might find that interesting.