Much of the advice is eminently sensible, whether you're prone to jetlag or not. Drink plenty of water, avoid alcohol, alter your diet, get to bed earlier before leaving, and keep as active as possible on board the plane. But what is the most important thing to do to stave off the effects of jet lag?

"The first thing is not to be exhausted before you leave Ireland, " says Dr Graham Fry, medical director of the Tropical Medical Bureau. "If you're exhausted before you leave and then get on a long-haul flight, that's not going to help."

That's easier said than done, as for most of us a long-haul flight is preceeded by weeks of planning and a rush to get everything done. At least those visiting America don't have to worry so much . . . eastbound travellers are much more prone to jet lag. Japan and Southeast Asia are the problem zones for Irish travellers, because the body's biological clock is less able to adjust to the shortened day that west-to-east air travel creates.

"The sun is going in the opposite direction, " says Fry. "If you leave Ireland and head towards Thailand, the sun is going from Thailand towards Ireland . . . so you're crossing [its path] and as you cross you lose more and more hours constantly."

Your communication skills, memory and decisiveness all slump, and if your brain is still on Irish time, you're at your lowest ebb between about noon and 2pm in Tokyo, just when you need your wits about you. Sleeping tablets or supplements that contain melotonin can help you regulate sleeping patterns while travelling. Melotonin is a natural hormone that helps your body clock stay in tune with daylight and sunshine.

"I've never taken it personally, but people I've spoken to . . . those in travel medicine, like myself, who have used it . . . say it is actually very good, " says Fry. "The problem with it is that it is not an easy dosage." Unlike regular sleeping tablets, the amount of melatonin you take depends on the length of your flight and whether you are travelling from east to west or west to east.

Regular sleeping tablets have their own drawbacks: "The problem with the sleeping tablets is the more you sleep on an aeroplane, the more likely you are to get deep vein thrombosis, " says Fry.

"You have to balance it. If you're on a 12hour flight, a short acting sleeping tablet that gives you four hours' rest is actually good, but one that gives you eight hours rest is not so good because that will increase the risk of a blood clot."

If you're disinclined to pop strong pills, there are no end of alternative remedies. Ben Mortimore, a conference organiser, drinks herbal infusions such as camomile or passionflower. "They don't necessarily put you to sleep, but at least they stop you worrying about it."

Michael Rodden, a theatre director, never leaves without a homeopathic concoction recommended by a doctor: "It's a combination of Cocculus 30 and Arnica. It doesn't always work, but if you believe it might then you're halfway there."

Other favoured potions include ginseng extract and aromatherapy oils. Company secretary Michelle Dalgliesh dabs essential oils on her wrists and temples depending on how she's feeling. "If I need to wind down, I use lavender and mandarin. If I need a lift, it's grapefruit and rosemary."

The best cure for jetlag is fresh air and direct sunshine, to let your body clock tune itself to destination time.

"Get out into the normal, natural sun, because that will adjust your own normal melotonin levels very accurately, " says Fry.

"Get to your hotel in your destination, get your bags unpakced and then go for a brisk walk out in the sun." Sunlight is believed to reset the body clock by telling the brain that it's daytime and not sleep-time, but according to textile executive Teresa Ferris, you have to be careful. "If you expose yourself to sunlight at your lowest ebb . . . when it's 4am back home . . . your internal clock will be fooled into believing it's later than it is, and whizzes forward by a few hours. I have to make the transition more slowly than that, otherwise I'm all over the place."

In contrast, importer Robert Sloman's first priority on his regular trips to China is to "fall into the rhythm of the place. Try to stay awake until it's time for the locals to go to bed, no matter how tired you feel. And then get up when they do. Any more than eight hours' sleep will leave you with a thick head."

So many theories, so many remedies.

Perhaps the only one that's guaranteed to work is to listen to other business travellers talking about it. Sleep can usually be assured within minutes.

Additional reporting by Niamh O'Doherty