Head rocking back and forth...

I've noticed, ever since I've started to be aware of myself as I fall asleep, that often when I'm falling asleep, like when I start to feel all drowsy and my body starts fall half asleep, that my head starts moving slowly back and forth, sideways.
I can stop it any time I want, but if I just relax and just "let it (my head) be", it will rock back and forth.
Only when I'm lying on my back, of course.

Has anyone else experienced this and if so, do you know why it happens?