In all honesty, I don't know and neither do most people.
I don't think that I nod my head, one way or another;it's a subconcious thing.
You were probably nodding while typing this.
Try to pay attention to your body language and see if you nod your head when you say no and shake your head when you say yes.
I'm always surprised to see it done, now, after having watched the body language experts talk about it. Frankly, I don't know anyone, in real life whom I talk with, face to face,who does do that.