An event where people know they'll be up far too late and will oversleep at this sleepover as a means of recovery from a hangover. This is the adult form of a childhood favorite.
"Hey, we're having an oversleepover tomorrow, you wanna come?"
"No, I gotta work the next morning."
Oversleptomania is a mood state characterized by sudden panic or fear attributed to oversleeping and realizing your entire day is screwed. Most severe when one realizes they are late for work.
Mr. Manager: "Bill, this is the 3rd time you've been late for work. You've missed a number of significant meetings."
Bill: "I'm sorry sir. I have a serious condition. It's called oversleptomania. I'm actually starting an affinity group for oversleptomaniacs in our company, in order to provide support and counseling."