When you are up so late or get so little sleep that you have the same symptoms as if you had been out at a bar/party pounding down the drinks... headache, stomachache, hungry, everything is loud, etc.
Man, I was at that party last night and didn't have a drink ... but was out so late I'm sleep hungover!
Not to be confused with National Hangover Day which is January 1st. This is the day after the 4th of July which is U.S.A.'s Independence Day. It is a result of too much partying the previous day.
Person1: Man, I feelawful.
Person2: You and me both.
Person1: Oh, yea thats because its National Hungover Day...
Person2: .....