ping for later
When I was in 8th grade, I had a dream that an evil looking demon was pulling the covers off of my bed. I awoke for real and couldnt move or speak for a second or two as the covers actually flew off the bed. I was scared sh*tless.
After I calmed down, I realized that I had fallen asleep reading with my knees raised. I think I turned in my sleep and the quilt slid off my bed. The rest was just my subconscious imagination accounting for the tug of the heavy quilt on my legs as it fell.
One night many years later, I heard of the Old Hag Syndrome. It seems a lot of people have had similar experiences.
See: http://paranormal.about.com/library/weekly/aa112000a.htm