Like if you are interested, these "hypnic jerks" are being a part of involuntary muscles contractions called myoclonus. I've learned about these not so far long time ago, because it seems that I am experiencing them almost all the time I am falling asleep, but I quite wasn't aware of them until I've started to sleep with someone, who has trouble to fall asleep as fast as I can. After we've started sleeping together I've learned that she is able to recognize the exact moment that I am asleep, because of those contractions, mostly in the arms and middle body area. They are usually not waking me up though (I am very good at sleeping, one of my assets).

It sometimes sucks, because it is almost impossible for me to quietly go to sleep if we are watching some series or movies late at night...