Hypnic jerk

A lot of times, when I'm falling asleep, I wake up with a start, as if I was falling. I googled it today and found out that, according to the unimpeachable Wikipedia, some researchers think that "as a subject falls asleep, their muscles begin to relax and cease working, causing the brain to believe that the body must be falling through air." So then you move around in the interest of self preservation.