Jet lag

When one's internal circadian rhythm is out of synch with external cues.  This is a phenonmenon more widely experienced since the age of jet travel.  If I leave Philadelphia at 7PM and arrive in Paris about 6 hours later, my body believes that it is 1AM.  However, for all of Paris, it is 7AM, and the day is just beginning.  It can take up to a full week to adjust one's internal clock to the new time, which means that even despite my enormous sleep debt, I may still suffer from jet lag at 3AM, when my body thinks its only 9PM. 

Source:

Dement, The Promise of Sleep