Jake of the Web

Sleep Paralysis; waking up in the middle of the night, completely unable to move - and with the certain feeling that something evil is in your room, on your bed, or even on top of you.

It turns out this is more common than one might think - nearly everyone experiences it at least once, but usually little more than that one time.

This happened to me once. I was terrified. It has not happened since, and despite the awful feeling accompanying it, I almost wish I could experience it again understanding what the phenomenon was.

http://www.wired.com/wiredscience/2009/08/sleep_paralysis/