Nodding is when the dream world and the waking world become one. The Latin "insomnia" best describes it. Whereas the English "insomnia" generally means the inability (or difficulty) to fall asleep, the Latin is more precise. It refers to a state in which the dream world (sleep) and the waking world (being awake) become one.
I've always thought that was interesting because Dido is "insomnia" in Book IV of Vergil's Aeneid, and there is also a reference made to papaver somniferum (the opium poppy). Was Vergil trying to say Dido was nodding? I don't know.
Yes, you can nod on oxycodone, but it's more likely with heroin. But, then again, everyone's different. While heroin may make most other people nod, you may find that something else will make you nod more, and vice versa.