The syndrome you have when you have gone through a holiday and no longer have a functional daily rhythm. A person with a Post Holiday Syndrome will then be awake at night and sleep at day. It may be in relation to the heavy partying a lot of people do when on holidays.
Guy: "Dude, I can not possibly go to school now! I haven't gone to bed before 3:00 AM in the last 2 weeks! All this partying has been killing me!"
Dude: "You're not the only one with a Post Holiday Syndrome..."