Jet-lag is a problem that is directly related to our 'body clock'. Almost every function in the body is influenced by a 'master' body clock which is located within the brain and sets our 'circadian rhythms'. These allow our body to predict changes in the environment and help to synchronize sleep and activity over a 24-hour period.

The most important factor in setting our body clock is the cycle of light and darkness. However, our body clock does not cope well with rapid and large changes in the timing of environmental signals like light, which is what happens when we fly quickly across world time zones.

'Jet-lag' is the term used for the symptoms you may feel while your body clock adjusts to time in the new location. Symptoms of jet-lag include feeling tired during the day in the new time zone, and yet experiencing disturbed sleep at night, feeling less able to concentrate or to motivate yourself, decreased mental and physical performance, increased incidence of headaches and irritability, loss of appetite and gastro-intestinal problems such as constipation.

Any journey, particularly if more than 3 hours, can cause discomfort and tiredness whether you are travelling by car, bus, train or aeroplane. You may spend long periods in a cramped position and there may be other stresses such as delays, unplanned stops or detours. These symptoms do not last long and often improve after a rest, some light exercise, or a shower or bath. You will usually experience symptoms of travel fatigue after any long-haul flight, but you will only suffer from jet-lag if you cross several (probably more than 3) time zones. You only cross time zones if you travel eastward or westward, so you won't suffer from jet lag if you fly more or less directly north or south, for example from the UK to Africa or from North to South America.

The severity of jet-lag depends on the number of time-zones you cross and, to a lesser extent, on the direction of travel. For most people, the symptoms are worse if they travel eastwards than if they travel westwards. The symptoms may also depend on whether you are a morning-type "lark" or an evening-type "owl". There is little evidence that jet-lag symptoms are different between men and women, or between older and younger people.

The symptoms of jet lag will naturally get better as the body adjusts to the new time zone. However, recovery may take 1-1.5 days per time zone crossed and there are a number of ways in which you can try to speed the process up. 

You can start the process of adjustment before you even leave home by changing the time at which you go to bed and get up. If your journey is eastward, for example from the UK to India, you could move your body clock forward by going to bed and getting up an hour earlier. Similarly, if you are travelling westward, for example from the UK to Canada, you could move your body clock backwards by going to bed and getting up an hour later. You should not attempt to re-set your body clock by more than 2 hours before you travel. 

If you have a choice of different flight schedules and arrival times, a late afternoon or early evening arrival might be better, since you can "use" the tiredness due to travel fatigue to help you get a full night's sleep in the new time-zone. 

Many travellers find it helpful to set their watches to the time zone at their destination as soon as the flight begins. Depending on the time of your flight, you should try to get some sleep during what would be night-time at your destination. The timing of meals is another cue that helps readjustment of the body clock, but most people have little choice in the timing of meals on the flight. Although the timing of meals can help to synchronise human circadian rhythms, there is little evidence that the content of meals, such as a diet higher in protein or carbohydrate content, speeds up adjustment to the new time zone.

If you are only travelling to a new time zone for a short period (less than 48-72 hours), it may be better to try and avoid adapting to the new time zone by staying on your home time. You should try to make sure that you eat and sleep at your normal times - if you are having difficulty getting as much sleep as you need, try to have a nap in the early afternoon after lunch, when you will naturally be sleepy. If you are travelling on business, try to arrange any important meetings for times when you will be more alert - during the local morning if you have travelled westwards or the local afternoon or early evening if you have travelled to the east.

The most commonly used 'medication' is caffeine in the form of caffeinated drinks, such as coffee. Other drinks such as tea or cola usually have lower levels of caffeine and there are also other energising drinks which may have much higher levels. Caffeine will help to combat any daytime sleepiness, allowing you to stay awake until the local night-time. However, it is important that you avoid caffeine in the evening, or you may then find it difficult to get off to sleep when you go to bed. 

Sleeping pills help to reduce jet lag symptoms by improving sleep, particularly if you are trying to get to sleep at a time when you would normally be awake and alert at home. Some sleeping pills can also cause daytime sleepiness and should not be taken if you plan to undertake activities such as driving, flying, scuba diving or skiing where this could be dangerous. They should be prescribed by your doctor, not be taken with alcohol and only used for a few days at a time. It is also better not to use sleeping pills during the flight as sleeping in one position in your seat for a long period might increase your risk of deep vein thrombosis (DVT). 

Many people find that they fall asleep more quickly after alcohol. Unfortunately alcohol has been shown to reduce the quality of sleep and so this is not a good way of combating the effects of jet lag.

Melatonin is produced naturally in the body and is important in regulating our body clock. It is often recommended as an effective treatment for jet lag but in many countries melatonin is not available 'over the counter' and has to be prescribed by a doctor. It can also be difficult to be sure of the amount and purity of melatonin bought in health food outlets or on the Internet.

A large number of studies have been undertaken to find out whether melatonin can reduce jet lag symptoms and many have shown that it can be beneficial. However, it is not clear whether these effects are simply due to it acting like a sleeping pill or whether it does help the body clock to adjust to a new time zone more quickly. The effects of melatonin medication are also affected by the body's natural cycle of melatonin production and it may be important to adjust the time the medication is taken depending on the direction and number of time zones crossed.

In many countries, including the UK, pilots and cabin crew are not allowed to use melatonin when they are on duty (which includes the days off when they are overseas) because of the unpredictability of its effects on alertness.

The most important factor in setting our body clock is the cycle of light and darkness. Not surprisingly, exposure to (or avoidance of) light is an important factor in how quickly you adjust to a new time zone.

Exposure to daylight has the most effect, but the best time to be outdoors if you are trying to combat jet lag depends on your direction of travel and the number of time zones you have crossed.

The table shows recommended times for avoiding or seeking exposure to daylight.