meh, I used to think that it was the opiates, but now, in hindsight, I know that it was a number of other factors that contributed to my marathon sleeping sessions -- namely, chronic fatigue syndrome.
I had a bad, bad, ridiculously bad -- terrible even -- case of mono. It was nightmarish, and the cherry on top of that shit sundae was an equally bad case of chronic fatigue, not uncommon after a bad case of mono.
Now, almost two years later, I've recovered somewhat and the chronic fatigue has gone away. But, at the time, I would sleep for 14 hours and wake up feeling exhausted. I still use the same amount of opiates if not more opiates now than before, and I don't experience the same fatigue as I did then.
But it would make sense that opiate abuse would contribute to one's sleepiness. However, in my experience, coming down off of stimulants resulted in more marathon sleeping sessions than with opiates.