It is early evening in New York and 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.
It is thought that, for every time zone crossed, 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 Britain's 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 per cent and reduce memory and communication skills. It can result in a lack of concentration, headaches, anxiety, slower reflexes and can also affect other biological functions, such as the kidneys.
So it is not surprising that there are a huge number of supposed jet-lag cures on the market, from pills to anti-jet-lag diets. 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 you could be confused and groggy.
It was recently suggested that Viagra might help alleviate jet lag. In a laboratory-simulated, six-hour leap in time zone, hamsters given the impotence drug recovered 50 per cent more quickly. 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 speed 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 Australia, it can be prescribed only by a GP and the Australian Medical Association's John Gullotta says there is still controversy about whether it works or not.
Is melatonin harmful? "Probably not," says Horne. "I don't take it because it doesn't seem to do much good."
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.
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. Last week, Boeing launched its 787 Dreamliner, an aircraft that gives passengers more oxygen and humidity, and is equipped with a lighting system that 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 Dijk. 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 Chris Idzikowski, director of the Edinburgh Sleep Centre and a consultant for British Airways, has devised a jet-lag "adviser" (find it at tinyurl.com/ycbscx), which will tell you the hours to seek light or avoid it.