The human body has in-built rhythms that organise body functions on roughly a 24- hour daily cycle. These rhythms can be influenced and adjusted to a large extent by environmental factors - the time on your wristwatch, whether it's light or dark, and changes in temperature. Rapid passage across time zones disrupts the natural rhythms, outstripping the ability of the body to readjust.

Few people who travel are unfamiliar with the resulting symptoms: general discomfort, fatigue, inability to sleep at the appropriate time, reduced concentration, impaired mental and physical performance, altered bowel habit and disrupted appetite and eating patterns - all are typical features of jet lag.

Adaptation

The body adapts to time changes at a rate of roughly one hour per day, so after a journey across eight time zones it may take up to eight days to adjust fully to the new local time. Many people tolerate westward travel slightly better than eastward journeys: westward travel results in a longer day which benefits those whose natural body rhythm is longer than a 24-hour cycle. Clearly a flight that does not cross time zones - north/south travel, for example - will not cause jet lag.

Further problems may also be experienced by those on medication that has to be carefully timed (people with insulin-dependent diabetes have to plan their insulin doses with care, for instance, and women on low-dose contraceptive pills may lose contraceptive protection if doses are missed or much delayed).

Children are often less affected by jet lag than adults, while the elderly may have great difficulty. Altogether, around 70 per cent of travellers are significantly disturbed by the symptoms. A wide variety of solutions has been proposed for those unfortunate enough to be badly affected.

Solutions

Melatonin : Melatonin is a naturally occurring hormone that functions in the body as a powerful internal signal of the approach of night. Melatonin is secreted by the pineal gland in the brain, in a pattern that normally follows a strict daily cycle. Melatonin secretion is suppressed by the presence of bright light.

In a number of placebo-controlled studies, small evening doses of melatonin have been shown to have a significant effect on speeding up recovery from jet lag - by about 30 per cent. Unfortunately, conducting trials on a large scale is complicated. The fact that melatonin is cheap and difficult to protect with patents means that pharmaceutical companies have had little commercial incentive to explore its potential in full. They are however working on melatonin analogues - synthetic substances that might have similar properties, but that could be patented.

Melatonin is available in capsule form in the USA as a food supplement, and may be bought off the shelf in health food stores. It has not been approved by the FDA or by drug regulating bodies elsewhere, and the situation has become further confused by recent extravagant claims that melatonin is a cure for almost everything, from impotence to old age. Consequently, regulatory bodies are reluctant to make it readily available. A recent informal survey of travel medicine practitioners attending an international conference suggested that over half of them had taken melatonin themselves, and more than 80 per cent were satisfied with the resulting beneficial effects.

Light exposure : Exposure to light suppresses melatonin secretion, and controlled exposure is known to alleviate jet lag. Various strategies have been proposed, some of which are difficult to understand and follow. At the simplest level, it is possible to use daylight simply as an environmental cure. More complex formulas claim to use precisely timed light and darkness to achieve dramatic jumps in 'clock setting'. Researchers at Harvard have attempted to patent various regimens of light exposure, a controversial move that will be interesting to follow.

The jet lag diet: With its 'jet lag diet', Ehret and Scanlon's book Overcoming Jet Lag (1983) became an instant bestseller, trading heavily on the claim that this was the strategy used by Ronald Reagan during his presidency. In this diet, protein and carbohydrate intake is scheduled in an attempt to enhance the synthesis of certain neurotransmitters within the brain at appropriate times. The claim is that, taken at breakfast and lunchtime, protein-rich meals that are high in tyrosine increase catecholamine levels during the day, while evening meals high in carbohydrates promote tryptophan for serotonin (and therefore melatonin) synthesis at night. In travel medicine circles, the diet has achieved a reputation for being almost impossible to follow - and so almost impossible to disprove.

It remains true, none the less, that meal timing is an important zeitgeber- a significant factor in influencing the body clock in its adaptation to a new time zone. So if it is not your habit to eat heavy meals in the middle of the night, resist the temptation to accept in-flight meals at times that are inappropriate to the time at your intended destination.

Widespread use of the drug Halcion during the late Eighties, taken halfway across the Atlantic (often with alcohol), resulted in an epidemic of short-term travel amnesia, in which travellers were unable to remember anything about the first few hours following their arrival.

Sleeping medication can also reduce fatigue during adjustment to a new time zone: it can help you get some sleep when you need to rest at what - for your body - is still an inappropriate time, and it can also help you sleep through the night. Zopoclone is believed to cause the least sleep disturbance. When using any drug, always take the lowest dose in the recommended range.

Melatonin also has a soporific effect, which some specialists have argued is the only explanation for any influence it may have on jet lag.

Planning your own jet lag strategy

Whatever your approach to jet lag, here are some tips to bear in mind:

1.-Flying westbound has the effect of lengthening your day. Avoid taking naps during the flight - this may prevent you from falling asleep later.

2.-Avoid alcohol, tea and coffee during and after your flight: all interfere with sleep.

3.-During eastbound overnight flights, such as the transatlantic 'red- eye', eat only a light meal before take-off and ask the cabin crew not to disturb you during the flight, so that you can get the maximum amount of sleep possible. Consider taking a mild sleeping tablet.

4.-If you can afford the luxury of time, take daytime flights where possible. Although they do not necessarily help you adjust better to the time difference, they cause least fatigue and loss of sleep, allowing you to arrive in best shape.

5.-Expose yourself to cues from your new time zone as soon as you can: reset your watch, eat meals and go to bed at appropriate times, and spend time outdoors.

6.-As body temperature falls naturally during the night, a common symptom of jet lag is feeling cold during the day: try a hot bath.

7.-Accept that there is bound to be some loss of performance when you first arrive in a new time zone, and plan your trip to avoid important business meetings for the first 24 hours after arrival; if you have to schedule a meeting on arrival, choose a time of day when you would normally - on home time - be at your most alert.

On arrival

On arrival at your destination, try to stay awake until night time without taking a nap. For the first night in the new time zone, a sleeping tablet is useful to help you to get to sleep at an unaccustomed time.

This type of occasional use of short-acting, mild sleeping tablets can be valuable and does no harm. Most doctors are willing to prescribe small quantities for this purpose. Possibly the most useful drug is zimovane, as it is short-acting and causes very little sleep disturbance.

Clearly, sleeping tablets should only be used on flights that are sufficiently long: it is pointless to take a tablet that will make you drowsy for eight hours when you are already two hours into a five-hour flight. Use the lowest dose that will work, and avoid alcohol. And remember that alcohol, sleeping tablets, fatigue and jet lag do not mix well with driving: too many people stagger off aircraft after a long journey and attempt to drive, when they are clearly not in a fit state to do so.

Whatever your approach, it is important to recognise that your performance is almost inevitably going to be reduced, so try to avoid important commitments and business arrangements for at least the first 24 hours after arrival.