These are called Hypnagogic Jerks and are completely normal. So normal, in fact, that many hypotheses state that it is a natural thing that occurs when the body is shutting down to go to sleep.
They are also sometimes called Myoclonic Jerks...I know there is a difference between the two, as they are not exactly the same.
http://www.medhelp.org/forums/neuro/...es/31231a.html
http://www.crhsc.umontreal.ca/dreams/TNmodeling.htm
http://encyclopedia.thefreedictionar...nagogic%20jerk
http://www.soundmedicine.iu.edu/arch...eepTwitch.html
What I find interesting is the brain's ability to fabricate an entire dream *and* sense of time lapse in the split second when this jerk occurs. For instance, the falling off a cliff dream that often goes hand-in-hand with these experiences.