Tired---moi?
Happens more often than folk care to admit to. For very obvious reasons. And not only 2-crew a/c. There exists a culture in many companies which actively discourages in-flight controlled napping. Not good, and yet here seems to be an example where in-flight napping may have gone wrong. Conjecture, I know.
I feel compelled to say that this incident could probably have been avoided by judicious use of the relief crew. (Although maybe they had only recently left the flight deck for their bunks). But with 2-crew ops on deep-night returns from UK/Eire to the likes of Cyprus or Egypt, the pressure is on to work safely and effectively as a crew.
Perhaps 411A has a point, but I know no honest pilot who can say they are never desperately tired in flight at certain moments. I know of no absolute solution to the problem.