It's just... wrong...
Thats what i mean, i don't mean you log off and you gain XP while you sleep, its for when you log off for the night, you'd have to be offline a certain amount of hours, say 8 (while sleeping) and then when you log in you gain a small xp bonus for when you kill stuff
And how do you imagine this can work in a multiplayergame? Player XY decides to take a nap - boom - the time gets shifted for all other players too?