WHAT CAUSES JET LAG?
Crossing time zones
The main, but not the only cause of jet lag is crossing time zones. Usually going east is worse than going west. Children under three don't seem to suffer jet lag as badly, as they are more adaptive and less set in their ways. Adults who adjust readily to changes of routine also seem less susceptible to jet lag. Those who are slaves to a fixed daily routine are often the worst sufferers.
Your pre-flight condition
If you're over-tired, excited, stressed, nervous, or hungover before the flight, you are setting yourself up for a good dose of jet lag. How many times have you heard travelers say "Don't worry, I'll catch up on the flight"? Well you don't. The wise traveller who wants to get the most out of a trip has a good night's sleep prior to departure.
The air aboard passenger jet aircraft is dry. To people who normally live in more humid conditions the change can be striking. The dryness can cause headaches, dry skin and dry nasal and throat membranes, creating the conditions for catching colds, coughs, sore throats or the flu. Drinking plenty of water helps, and some frequent flyers take a bottle of water with them. Some airlines supply water frequently to passengers, but others only have a small water fountain near the toilets. Coffee, tea, alcoholic drinks and fruit juices are not recommended. Water is what your body wants.
At a cruising altitude of near 30,000' the aircraft is pressurized to near 8,000'. Unless you live near 8,000' and are acclimatized to this pressure you may suffer from swelling, tiredness and lethargy.
Providing a constant supply of fresh air in the cabin costs the airlines money, and some airlines are more willing to oblige than others. The air supply in business and first-class is often better than in economy class. A lack of good air helps make you tired and irritable and can cause headaches. Sometimes if you ask the flight attendants to turn up the fresh air they will do so.
The impact of alcohol on the body is 2-3 times more potent when you're flying. One glass of wine in-flight has the effect of 2-3 glasses on the ground. Add this to the other problems mentioned here, and you can get off the plane with a huge hangover that simply compounds the effects of jet lag.
Food and drink
Airline coffee and tea not only tend to taste awful - they have a higher than usual caffeine content and are abrasive on the stomach. Orange juice is also abrasive if you are not used to it. If you don't normally drink really strong coffee, tea or orange juice, don't try it while flying. Also go easy on the frequent meals served in-flight. You don't need them. And sitting in a cramped position puts extra pressure on your stomach. Also beware risky foods served on some airlines in certain parts of the world, including salads and cold meat and fish. According to WHO, 50% of international travellers get stomach problems, so dietary care is important while flying.
Lack of exercise
Lack of exercise is one of the worst aspects of longhaul flying. It makes the flight uncomfortable and sets you up for a longer period of jet lag afterwards. Do stretching exercises in your seat, especially for the legs, and if possible go for walks up and down the aisle. If you have a spare seat next to you, try to get your feet up. Get off the plane whenever possible at stopovers and do some exercises (don't worry what others think). If there is an opportunity during a ground stop, take a shower - it freshens you, tones the muscles and gets the blood moving again.