What causes jet lag?

Circadian rhythm. We have an internal clock that is aligned with the 24 hr light cycle where we live. When we travel, especially west to east we are not aligned with the light in that time zone and our internal clock won't let us fall asleep sooner. Going west we can generally stay up later, but will have a tendency to wake up at our normal time which will be very early for the time zone we're in.
Clock mismatch. It is a mismatch between your internal clock and the time in the place you are in. It is worst when you travel east.
Travel East or West. Jet lag is caused by a mismatch between your internal clock and the external world. Your internal clock is set and reset every day by sunlight and melatoinin. To combat the fatigue, malaise, and nausea which go along with jet lag try to reset your internal clock: get out into the daylight in the morining and evening. You can use melatonin (available over the counter) before bedtime too.