Jet Lag is a debility similar to a hangover. Jet Lag derives from the simple fact that jets travel so fast they leave your body rhythms behind.
Our biological clocks are synchronized to a 24-hour period. Our internal clocks drive our circadian rhythms, which anticipate dawn and dusk, and control everything from blood pressure to how hungry we are. When we fly to a different time zone, (or work night shifts), our internal clocks go out of sync.