Results from disruption of circadian rhythms in the body. It is much worse going east than going west, and this is why:

Westbound It takes about 75 minutes to fly one time zone - give or take 10 minutes depending on how straight you fly. When you fly west, you usually end up landing only shortly after the time you took off. This means stretching out your day with additional hours; a five hour flight westbound - four timezones - will make your day about 28 hours long. No big deal.

Eastbound Flying east is another story. For the same reason as above, you lose one hour for every hour of flight. After a five hour flight across four timezones eastbound, the time is nine hours after the the time you took off. This screws your day up big time, usually by making the day about 40 hours long.