"C. Sleep paralysis, as defined by David Hufford, is "a period of inability to perform voluntary movements, either when falling asleep or when awakening, accompanied by conscious awareness." Such paralysis during sleep is normal. But what may constitute a sleep disorder is bad syncronisation between the sleep state and paralysis, which can result in the subject being paralysed and fully awake. A sensation of heavy pressure on the chest often accompanies sleep paralysis."

From: http://www.generation.net/~valmont/ivtq/ivtq12.html

This isn't exactly what I was talking about but it's interesting (I was thinking of the Henry Fuseli painting, "The Night-Mare"). A version of that can be seen here:

http://www.illusionsgallery.com/nightmare.html

Sorry if these don't come out blue and clicky. I don't know how to do that.