Jet lag, of course, is what happens when the biological clock gets out of sync with the chronological clock of a new time zone. Your body is geared for sleep at a time you are expected to be awake, and vice versa. Most people say they get more jet lag flying east than west. They find it easier to cope with a longer day than a shorter night.

The reason is that the circadian rhythm (the rhythm based on a 24-hour cycle) has a natural tendency to run at a sleep-wake cycle of 25.2 hours. So if you fly west, you're gaining on yourself the whole time, and the clock has to run a bit faster; whereas coming the other way it has to run slower, which it seems harder for it to do. Westward flights produce premature awakenings plus sleepiness in the evening. Eastward flights result in difficulties staying asleep and morning sleepiness.

Light is the main trigger, or synchronizer, of the clock, although social cues, like mealtimes, also affect circadian rhythm. Most people adjust at the rate of one time zone per day. This means you would need a week to properly adjust to a flight from, say, Europe to the Far East.

Ask a dozen frequent travellers how to cope with jet lag, and you're likely to get a dozen different answers, from elaborate diets, in-flight aerobics, and aromatherapy, to seeking nirvana through meditation (or medication).

Conventional wisdom says you should adjust as fast as you can to local time by re-setting your watch and thinking in the destination time the moment you get on the plane. Expose yourself to bright light - especially sunlight - depending on which direction you are travelling. From London, for example, travelling to Tokyo, try to get out in the sunlight later in the day when you arrive. Going west to Los Angeles, try to get a good dose of sunlight as soon as you arrive. The idea is that light suppresses melatonin - a sleep-inducing hormone secreted in the late evening, and thought to be the master synchronizer of circadian rhythm.

However, recent studies suggest that the best strategy for trips of up to 48 hours is not to adjust but stay on your home time. This is the advice that many airlines are giving to their air crew members. If you are staying less than 48 hours and have to work when you come back, it's important to stay with your home rhythm and try to eat sparingly. Wear sunglasses at times when it's dark at home, get artificial light to simulate sunlight when it's supposed to be daytime there, and don't sleep when you shouldn't.

Going west, go to bed as soon as you can and get what doctors call "anchor sleep", for, say, five hours. You can still go out for late dinner. It's important to get plenty of sleep before you leave home. Napping can help.

But you have to compromise between what is going on where you're going and at home; you'll find there's an overlap of some four hours. If you're going for longer than 48 hours, adjust to local time. It's a strategic decision.

The real challenge is travelling to the Far East with a 7-9 hour time shift. Says one old Asian hand: "I've tried leaving Europe in the morning and getting to Hong Kong first thing in their morning; and I've tried leaving Europe in the evening and arriving in Hong Kong at 3 or 4 o'clock the next afternon - by far the best. There is nothing worse than arriving at 7 o'clock in the morning, when you've been up all night and you're confronted with a full day ahead of you. You can't even check into your hotel for another five or six hours. Whereas if you arrive at about 3 o'clock in the afternoon local time, you can either have a quick nap, or even better hold off until 9 o'clock local time and go to bed.

"It's best to stay on your home schedule. Your day starts later and finishes later - so plan your business meetings for the afternoon and evening. The worst part is not so much the time change, but how much sleep you really get on that long 12-14 hour eastward flight. Beyond that, even if you try to adjust immediately to the local time, it catches up with you by about the third day.

"Therefore in a perfect world, if I am flying out east for more than 48 hours, I would fly on a Wednesday so that on the third day, when the loss of sleep hits you, you are on a weekend and can recover without losing any business."

The moral is that next time you face a midnight lunch appointment, make sure you get a proper day's sleep!

Do's and Don’ts • Do try staying on your home time for a trip less than 48 hours. But adapt as quickly as possible to local time if your stay is longer • Do check out the jet lag calculator from travel health specialists MASTA to try to re-orientate yourself • Do try when travelling east to schedule your arrival for late afternoon. Have a meeting, go to bed late, and start late local time next morning. Travelling west, try to have meetings from early morning through early afternoon. Then sleep on your home time • Don't expose yourself to sunlight when it is supposed to be your normal night • Don't skip meals, but make them smaller than you'd like, especially during your normal night. Avoid alcohol, tea and coffee on the plane