Jet Lag : Extreme tiredness and other physical effects felt by a person after a long flight across several time zones. Jet lag is caused by a flight's direction, not it's length.
There is no way to completely avoid jet lag, no matter what anyone may tell you, but there are many ways to combat it and help your body adjust to a new time zone.
Depending on the direction you're headed...slowly adjusting to your new time zone prior to actually departing may help. Some people, however, find it works best to go to bed earlier for a few nights before you leave. This, as well as snoozing in the air while resisting temptation to binge on inflight entertainment should ensure you're well rested for the journey ahead. Blue light can often irritate your eyes and disrupt your body clock, so switching off is probably a good idea anyway.
Sometimes lavender oil can help to calm you if you're one to get antsy in the run up to a journey.
If at all possible time your flights to help with the adjustments. This way, you can avoid napping and jump straight into your new time zone. Take it easy on the day you arrive so that you can take advantage of youir trip at a leisurely pace and establish a routine in sunc with the local time.
Many side-effects of jet lag are the result of dehydration, so avoid alcohol, coffee, tea and other caffeinated beverages. Remember to drink plenty of water during your flight. Many airports have drinking fountains, so consider bringing a refilable water bottle with you to help you stay hydrated. An average three hour flight can help you shed 1.5 litres of much needed water!
Eat lightly during your flight and forego rich or exotic foods for the first few days of your trip so that you can use your energy to adjust to your new surroundings rather than to digest unfamiliar food. The healthier you eat at the begining, the longer you'll last on the trip!
Exercising on a long flight will help alleviate common discomforts such as bachaches, swollen legs/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.
Your internal clock depends on sunlight, getting some sun helps keep your melatonin cycles in balance. You can also ask your doctor about the option of melatonin suppliments, which can be a great natural aid for sleep while abroad.