I have experienced the phenomenon described in the following link while renting a room in an old supposedly haunted farmhouse from a family during college. Never happened to me before nor since. Very scary.
http://paranormal.about.com/library/weekly/aa112000a.htm
Happened to me once. Could not move a single part of my body, not even my lips. I shook off the fear and sense of dread that was all over me, then mumbled out the Lord's Prayer and fell unconcious. Woke up a few hours later just fine.
Has happened to me 3 times.
It is a natural sleep induced paralysis.
There is stuff all over the internet about it.