In the non-airline multi-crew ops I have flown it has been pretty standard to take a nap when tired. Amazing how it freshens you up. "Wake me up at top of descent" seems eminently reasonable to me, as long as the other pilot isn't feeling dopey himself. I will refrain from posting piccies of pilots asleep at the controls!!
Let's face it, over half an hour in the cruise is like over half an hour on the motorway - those eyelids can start drooping! And once they do.....