Jet lag can strike when you least expect it especially after travelling for a long time or crossing multiple time zones. Your internal clock and normal body rhythms get confused which makes you feel tired, sluggish and even dizzy. Not exactly what you had in mind when you took a holiday! You're not helpless, though -- eating right, drinking plenty of water and getting as much shut-eye as possible can straighten you out and get you right back on track or even prevent the jet lack from kicking in to begin with.
During your flight, drink as much water as possible. Keep it coming - pick up a large bottle at the airport after clearing security and ask the flight attendant for a cup every time she walks by. On that same note, avoid alcohol and caffeine unless you really need a cup of tea or coffee to keep you up. Flying makes you dehydrated to begin with, and substances that add to that effect will just make you feel so much worse!
Next, avoid heavy meals and foods in flight. The altitude and changing eating schedule can upset your entire digestive system, so stick to lighter fare. Avoid salt and fatty foods as much as possible, as salt will cause you to retain water and bloat, while fat will sit in your stomach longer and take longer to digest which can enhance the effects of an already shifty digestive system. This follows even when you arrive at your destination. It may take you a few days to get used to different foods and schedules of eating if your internal clock is set to a different time, so keep it light and fresh and graze throughout the day if you can.
Lastly, try to get as much shut-eye as possible on your flight, and expose yourself to as much light as you can when you leave the plane -- sunlight can help you reset your internal clock. If you still can't get adjusted to the new time where you land, try taking natural sleep remedies like valerian or melatonin to help you doze off at the right time.
Travelling can be so much fun, but only if you're feeling good - it's exhausting too! Give your body all the help you can so you're ready to hit the ground running and enjoy your time away to its fullest.