Sleeping passenger etiquette, what do you do with them?

If you are on a long train or jet ride and the passenger sitting next to you falls asleep and their head comes to rest on your shoulder what do you do? Do you leave them be so long as they are not cramping your style, wake them up and alert them that they fell asleep, if you previously did not disturb them but they started drooling on you do you wake them or move them over and say nothing as to not make them feel bad? What is your policy on sleeping passengers?