May, this sounds a lot like what I had post-op with my acdf and what another poster has had post-op, too. With me, I didn't have my head jerk involuntarily, but the rest of my body (arms, legs, torso--which moved my head, but didn't originate w/ it). Like you, this would happen to me when I was resting, falling asleep, or actually asleep. In my case, the neurologist said it was a reaction to the morphine, anesthesia, and meds I had post operatively. He called it myoclonus and said mine was specifically hypo-something-clonus. He gave me a Rx for an anti-seizure type med called, clonazepam that's suppose to have a 'calming effect' on my nervous system (to counter the reaction if I have it with the meds again -in my next surgery). If your involuntary jerking is occuring unrelated to Rx drugs (even if it IS, actually), I'd get in to see a neurologist right away --and also ask them about clonazepam for you, too. I hope this helps and good luck to you!
Meghan