Clocks out of synch. Jet lag is a mismatch between your internal clock and the external world. This happens when you travel east or west (less so with westbouond travel) often you will feel fatigued, have poor appetite and may have trouble sleeping. Sunlight and melatonin set your internal clock. To treat it get out into the sunlight, eat at regular times, and take melatonin if you have trouble falling asleep.
Circadian rhythm. We are aligned to the pattern of light where we live. When we quickly change time zones our internal clock needs time to adjust to the new light pattern. It's easier to travel west because we can stay up later easier than we can fall asleep earlier which happens traveling to the east.