Just before falling asleep, why does my body sometimes "jerk"?!

Just before falling asleep, why does my body sometimes "jerk"?

Twitches while falling asleep are called hypnagogic myoclonus, myoclonus being any sort of involuntary muscle spasm and hypnagogic referring to sleep. The twitches occur during very light sleep as the conscious brain gradually relinquishes control of the motor functions. Often they're accompanied by a sense of falling, or the feeling that something is flowing through the body, and sometimes people will experience vivid dreams or hallucinations.

It's not known exactly what causes the twitches, but they appear to be associated (although by no means invariably) with (a) anxiety and (b) some faint stimulus, such as a noise. The twitches have been induced in test subjects who were instructed to push a button whenever they heard a low tone. When, as usually happened, the subjects nodded off after a while--you know how exciting psychology experiments are--the tone would often cause a subject to twitch after a lag of a few seconds.

It's conjectured that the subjects consciously knew they were supposed to stay awake, that they fell asleep anyway, and that the tone jarred the semiconscious brain into trying to scramble itself into action again. That would explain why you experience the twitches during boring lectures. It's thought that at times the stimulus can be purely involuntary, such as a dream.