Sleeping in public

It is great fun watching an audience of silver haired ladies and gentlemen listen to a post-prandial lunchtime talk. The speaker was good and lively, the subject was of interest - otherwise why would they have bothered to come and pay their £20? And yet, surveying the scene from the edges, one could see eyes shutting intermittently, followed by well-closed eyelids and soon after, heads began to drop forward! Even if you couldn't see the faces, you could tell what was happening because lolling heads would suddenly jerk up, as spouses dug the dropper-off in the ribs or some internal watchdog woke the sleeper.

The cinema or theatre are worse because, as the auditorium is in semi-darkness, the sleeper finds it easier to stay asleep and if you are unlucky, he is sitting next to you and gently and imperceptibly leans his head on your shoulder! Snores are worse because more more people than the immediate neighbours hear them. What is the etiquette for such circumstances? Can you prod a total stranger and, in these days of gratuitous violence, might your culture loving neighbour turn into a knife wielding maniac?
Of course, if the sleeper happens to be your nearest and dearest, you have the additional task of recounting all the bits he missed.