On my way back to Srilanka, i happened to doze off in my Northwest flight from Seattle to Japan. The air hostess in the flight didn't wake me up and i missed my dinner in the flight. So i have seen in many flights they have different policies when a passenger is sleeping in the flight during meal time. In Northwest they give priority to sleep and they make sure you don't get disturbed during meal time. So if you happen to sleep, don't think the air hostess is going to wake you up during meal time.
Srilankan airlines had a nice policy of tapping you a couple of times and if you don't budge then no meals for you :). But a couple of years back when i was travelling, I think it was Qantas airlines, they gave a sticker that said "Do not disturb". If you stick that in your seat then that means you want to forego your meal for sleep and if not they make sure they wake you up during meal time. I wonder whether they have an universal code of conduct on the rule of waking up the passenger during meal time ;-)