It happened to me once ( I was a Cabin chief), after a 24 hours LH layover following a 2 sectors O/B, and during the 2 sectors I/B night flight, to find both pilots asleep. The FO was on his planned nap hour, and the CPT just fell asleep. I was embarrassed for him, and I asked him loud if he needed some coffee too wake him up.
Of course you must know as a FA, that those things may happen when pilots (and FA)are treated like robots and not human beings, and check on the FD more often (thing that helps you too to remain awake!).