No official penalty has ever been specified. Most GM's that I know rule that "waking up" takes a move action. That plus the obvious implication that the opposition gets a suprise round ought to put the sleeping character at a sufficent disadvantage.
You may also want to consider a -5 penalty to innitative.
from the sleep mastery feat, it says (quoting from memory): you may make listen checks as if you were awake, and if you succed in that check you may become fully awake as a free action. so, if one of the benefits is becoming fully awake, there should be penalties for that. argo suggestions could are good enough disadvantages as he says. another question comes to mind from the same feat, and i am pretty sure i have read this somewhere, though i don't remember where: when you are asleep listen checks are made at -20, is that right?