When you wake up after a heavy night of partying feeling pretty good on account of still being drunk; as opposed to hung over.
Having a left over can often be translated into a bearable day (given one has nothing to do) through the use of marijuana, greasy breakfast burritos, or more alcohol followed by a nap. The hair of the dog is just enough to stave of the hang over until you can deal with it in your sleep.
Late-night pot smoking helps the body lean towards a left over the next day. Don't ask me why, it just does.
Frank: "Dude, I feel like shit after drinking Budweiser last night."
Linus: "Man, you shoulda hit that joint. I have a total left over and am about to go to IHOP!"