Why does my whole body jerk before I fall sleep?

Hypnagogic 'jerk' 70% of people have had these: a hypnic jerk, hypnagogic jerk, sleep start, or night start, is an involuntary myoclonic twitch which occurs during hypnagogia, just as a person is beginning to fall asleep, often causing them to awaken suddenly for a moment. They are often accompanied by a 'falling sensation' possible causes are: anxiety, caffeine, stress, and strenuous activities in the evening.