Drunk, Sleeping – They get drunk, sit, get comfortable then fall asleep and stay asleep.
What else you might try – Usually not a problem provided there is a place for them to sleep in peace, if they don’t snore too loudly, and have eventual transportation home. It’s also a good, if somewhat immature, idea to have markers handy and placed next to Sleeping Beauty. Your other guests can scribble their appreciation or other thoughtful suggestions on this person.