It is most likely the oxycodone in the percocet
that is making you sleepy. Unfortunately it is just a side
effect of the medication for some.
I take hydrocodone and as the dose wears off I become sleepy. Sometimes if I am idle I can doze off for a few minutes. I find that caffeine helps. As soon as I take my next dose the sleepiness subsides. I am not positive but I think this is a fairly common side effect.
I hope this helps.