It is early evening in New York - and as the city is getting ready to go out, you feel ready for bed. Or the morning sun hits your Beijing hotel room, but you think it is midnight and have barely slept. Nearly a week into your holiday and you feel more tired and fractious than when you arrived at the airport.

For every time zone crossed, it is thought it takes a day to recover from jet lag. "You are taking your time to another time zone and realising that your body clock doesn't fit. It does take a few days to adjust," says Professor Jim Horne, director of the Sleep Research Centre at Loughborough University and author of Sleep Faring: A Journey Through the Science of Sleep.

It takes longer to adapt if you travel east, because your body clock - or circadian rhythm - finds it more difficult to adjust to a shorter day, than a longer one. "If you fly west, you will find it difficult to stay awake but easy to wake up," says Professor Derk-Jan Dijk, director of the Surrey Sleep Research Centre. "If you fly east, typically it is the other way around."

Jet lag does more than make you feel a bit grumpy: a British Airways study showed that it can impair decision-making by up to 50% and reduce memory and communication skills. It can result in a lack of concentration, headaches, anxiety and slower reflexes, as well as tiredness, and can also affect other biological functions, such as the kidneys (which is why you may want to go to the loo at odd times). "You could have gastro-intestinal problems because there are many clocks in the rest of your body, not just in your brain, which can be disrupted," says Professor Dijk.

So it is unsurprising that there are a huge number of supposed jet lag cures on the market, from pills to anti-jet lag diets (which seem to involve a complicated plan of eating protein and carbohydrates at different times). But what really works? First up, sleeping pills aren't recommended. Although they can help you go to sleep at times your biological clock wouldn't normally allow, if you have to wake up before they wear off - which can often happen on a flight - you could be confused and groggy. There is also a worry that lying immobile for long periods could put you at risk of developing deep-vein thrombosis.

It was recently suggested that Viagra might help alleviate jet lag - in a laboratorysimulated, six-hour leap in time zone, hamsters given the impotence drug recovered 50% more quickly. Professor Horne remains unconvinced: "It might help globetrotting hamsters, but at the moment, there isn't any evidence to suggest it works in humans."

Another popular jet lag "cure" is melatonin pills, which are supposed to help, primarily by speeding up your internal clock. Melatonin is a hormone that is naturally produced in the pineal gland in the brain, which helps regulate the body clock. In the UK, it can only be prescribed by a GP and, even then, some may be reluctant because it is unlicensed. It can be bought over-the-counter in the US (although it hasn't been approved by the US Food and Drug Administration) where it is sold as a nutrition supplement, or over the internet, although you can't ensure that melatonin is what you'll be getting.

Is melatonin harmful? "Probably not," says Professor Horne. "But I'd question the quality of the pills you can buy over the counter. I don't take it because it doesn't seem to do much good. I'm not against it, but the problem is that most people don't know how to take it properly. "It tricks the body into thinking it is dark and not time to get up. So if you wake up at 4am when you are away, and are very alert, take it then and avoid putting the light on. If you're flying back from the States, for instance, and it is dark when you get home but you are as bright as a button, take it at bedtime. Whenever you take it, avoid bright light - natural or artificial - within two hours." If you take it at the wrong time, melatonin could even make jet lag worse.

Dr Chris Idzikowski, director of the Edinburgh Sleep Centre and a consultant for British Airways, says dosage is important. "I prefer lower doses - around 1mg. If the brain is swamped, the effect can be confused and it starts working against itself. If it doesn't seem to have any effect after two or three days, I would stop taking it."

Melatonin has been found in some varieties of grape skins used to make red wine, including merlot and cabernet sauvignon - which is thought to explain why drinking red wine can make you sleepy - although all experts agree that you should avoid alcohol while flying as it will dehydrate you (which, in itself, can cause symptoms of weariness).

So if there is no quick-fix pill, what else should a tired traveller be doing to alleviate jet lag? Adjusting the times when you go to bed and get up before you fly, could help, as could making sure you don't have a sleep deficit before you travel, although this isn't always convenient (or indeed, likely).

It is generally agreed that you should get into your new local time as soon as you get on the plane - changing your watch and eating small meals to coincide with new meal times.

Meanwhile, some airlines are now taking jet lag seriously. Next year, Boeing is due to release its 787 Dreamliner, an aircraft that gives passengers more oxygen and humidity, and is equipped with a lighting system which helps with the switch to local time by simulating the light and colours of sunrise and sunset. In fact, light seems to be the most effective way of combating jet lag. "Our biological clock synchronises to a 24-hour cycle by light," says Professor Dijk. The retina, at the back of the eye, transmits light signals to the hypothalamus, the region of the brain that controls a number of biological cycles.

By exposing yourself to light - indoor light does work, but outdoor daylight is three times more effective - you can speed up the time it takes to reset your body clock, but the trick is knowing when to get light exposure.

To help, Dr Idzikowski has devised a jet lag calculator (find it at ba.com), which will tell you the hours to seek light or avoid it.

This should reduce your experience of jet lag to a couple of days, rather than a week - but getting the right light exposure depends on what your usual wake-up time is, where you go, what time you arrive, and how tiring the journey has been. Is it really worth the trouble? Yes, if you are going away for a couple of weeks."If you're crossing two or three time zones, it's not really worth trying to do anything with your internal clock," says Dr Idzikowski.

"If it is a business trip for two or three days, I would just advise to manage your sleep and work out when you need to be alert for a meeting."