It can take five days to feel normal again after a long-haul flight. What with daytime sleepiness, nightly insomnia, loss of appetite, clouded thinking and poor co-ordination, this can seem like a long time. It's worse if you are sleep-deprived before you travel, cross more than four timezones, get dehydrated on the flight by drinking alcohol or if you are travelling east, which we find harder to tolerate than going west as the body clock copes better with being asked to stay up longer.

Jet lag is caused by disrupting your circadian rhythm, the internal body clock that regulates sleep and waking. This clock is a tiny group of cells in the brain: the hypothalamic suprachiasmatic nucleus. It's controlled by light and dark and the hormone melatonin, which is produced when it gets dark and controls our body temperature while we sleep.

Melatonin can be made synthetically and in America is available as a herbal remedy over the counter. In the UK it is classified as a medicine and is only available to people over 55 with insomnia. But would it help jet lag? Or should you try sleeping tablets to get to sleep and stimulants such as coffee to keep awake?

The solution

A systematic review of research by the Cochrane Collaboration revealed that melatonin can be taken to reduce jet lag when crossing two or more timezones. Between 0.5mg to 5 mg of melatonin, taken daily at bedtime, helped people to get to sleep faster and better (particularly for the higher dose), as well as reduce sleepiness during daytime.

Melatonin works better the more timezones are crossed and for travelling east more than west. However it is not safe for everyone and people with epilepsy or on warfarin should not take it. There is some evidence to suggest that if you travel west, but are only staying for a couple of days, it is best to stick to your home timezone to reduce jet lag, otherwise you should adopt the local time as soon as possible.

If you are travelling east, it helps to stay in the dark for at least three hours after arriving to try to reset the circadian rhythm. If going west, get out in the daylight.

Sleeping tablets are often used to get back into a waking and sleeping cycle but the evidence is not clear that they work. Caffeine reduces sleepiness but makes it harder to fall asleep at night. And the really bad news is that research suggests it doesn't even help to sleep on the plane, unless you're flying when you'd usually be asleep.