Hypnic Jerk

The neurological reason for hypnic jerks is not fully understood, although the two dominant theories suggest that as a subject’s heartbeat and breathing slow down, hypnic jerks occur as a natural part of muscular transition; or 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. It is thought that this causes people to thrash their limbs in an attempt to catch something or turn oneself upright. (via Wikipedia)