A nod is when your mind is transitioning back and forth, fairly rapidly and without much control, between a drowsy wakeful state and a seemingly lucid dream-state. You don't notice yourself going into the nod, 30 seconds to a few minutes later you suddenly "wake up" from it, wonder how you nodded off so easily, and proceed to start nodding again, with the most blissful unbelievable euphoria coursing through every inch of your body.