What causes jet lag?

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.