It is a fun way to say "Good morning." or if it is not morning, "welcome from your long sleep."

All of the above. Plus we have another saying. 'Oh, you're back from the dead!' As an exclamation. We say this when somebody has been ill with the flu or a sickness and we haven't seen them for a while.

This has been used after I have travelled to strange, foreign countries. When I returned from a long trip someone said, Welcome back to the "land of the living"....like this is where I belong. It is just an idiom saying this is the real world (for me).

Yes, it's said to someone who's just woken up - maybe even sleeping off a night of drinking? If you're in a deep sleep, you're "dead to the world".