After crossing several time zones, many travelers suffer from "jet lag." Although there is no way to completely avoid jet lag, there are a number of ways to help your body adjust to a new time zone.
Get as much sleep as you can during the flight. Keep yourself hydrated, avoid drinking alcohol, coffee, tea and other caffeinated beverages and drink plenty of water during your flight.
Exercising on a long flight will help alleviate such common discomforts as backaches, swollen legs and feet and general fatigue.
Stretch at regular intervals and walk up and down the aisles of the plane from time to time to prevent dangerous blood clots from forming.
Get in synch with the new time zone as soon as you can. The headache, light-headedness and nausea associated with jet lag should lessen significantly when such measures are taken.