Opioids are actually well-known for producing this side-effect. It's a little difficult to explain what a "nod" precisely entails to your average person who has not experienced it. People assume that Opiates, being Narcotics, put you to sleep, as that's what they seen and it's what they are told, but users know that the reality is actually much different. Opioid sedation is not universal or complete. You'll often find that a heavily intoxicated user can actually maintain conversations, perform complex tasks et cetera following stimulation, but once this constant input of sensory stimulation has ended, they can immediately revert back to unconsciousness.
Those with experience in dealing with Opioid addicts are probably aware of the insane ability of an intoxicated individual to carry on a complex conversation about various topics only to nod out mid-sentence. You go back to what you were doing for say, 20-30 minutes, only to have said individual spontaneously restart the conversation from the very point in the sentence that he had nodded out during. He then asks you why you're being so quiet, insults your social skills/class. You're frustrated, but ultimately resume the conversation, trying to remember what the hell you were talking about, only to have the dude revert to unconsciosness 30 seconds in.
The nod is a complex state of intoxication between wakefulness, dreaming, unconsciousness and consciousness that is going to be nearly impossible to actually describe or chart without a system of metrics to understand the progression of the individual e.g. 02, EEG, Pulse/Pressure et cetera. Definitely not uncommon.