I've had three or four experiences in my life of napping (it never happens at night, oddly) and thinking that I've decided to get up, but can barely move -- it's like my entire body is made of lead, and dragging myself off the couch is the hardest thing I've done in my life. Then I realize I haven't actually gotten up, so I try again, and again, and again . . . and eventually I actually wake up, and I'm fine. Not your "encased in stone," but almost certainly the same neurological experience, interpreted slightly differently.

And yeah, I agree -- I'd love to know more about the neurology behind such things.