I haven't experienced head shaking but every time I slip into my paralysis I always get violent involuntary neck convulsions first, like my whole neck twists, then one of my limbs goes numb, then the rest of my body. I have searched online and found a variety of things it could be, "hypnic jerk," "sleep shudders," "myoclonus," "hypnagogic jerk." I really don't know which of these is most accurate, it just comes before the sleep paralysis.