I did this ALLLLL THE TIME! But I never felt guilty. (I later found out that I have sleep apnea and without treatment, falling asleep at the drop of a hat is pretty common with this condition.) I did applogize to God for falling asleep, but I was never scared he would "smite" me or something. I think my view of Jehovah was a bit different than the one that the rest of the congregation had. I was never scared of Him. He knew what was going on in my life to make me so sleepy. So, if I fell asleep during a prayer, I'd just finish it the next morning, and insert my apology at the end of that prayer.