I was strung out on smack and other strong opiates for over 6 yrs so take that for what you will. I never slammed anything though, so that changes the way you should interpret the following.
The nodding is not indicative of an overdose in my experience. Whenever I have OD'd (yes you can overdose and not die) on opiates, I've always come really close to a nod but could feel a distinct difference (and had an intuition that I had done too much) and usually would experience unconsciousness fairly quickly and not really nod. Cold showers work well for staying "awake" and I always had naloxone on hand which thankfully I never had to use but I did get close a few times. As long as there is still ventilation, one of the main problems can be asphyxiation which seems to be what is the main cause of death in OD's it seems. Obviously, as long as you can stay awake, you will be fine but beware of vomiting and the position you are in. I've had times where my respiration rate was literally 3-4. It is a scary sensation but you will be okay (hopefully).
Try not to go to sleep if you truly are worried. When I was in your situation, I would notice the same things and not allow my self to sleep. I very well might have saved my life because of this. When it comes to things as potentially serious as this, there's no reason not to err on the side of caution.
