What jet lag does to your body - and how to cope

Perhaps we shouldn't be surprised that flying halfway around the world faster than our grandparents could have dreamt of leaves us feeling terrible.

The most obvious symptom of jet lag is fatigue. Anyone who's taken the red eye from the US to Europe and then attempted to stay awake at work the next day will know all about it. Then in the middle of the night your body clock thinks it's time to play and despite your fatigue, you are now wide awake.

But of course jet lag is not only about feeling exhausted. No, it has profound effects on the body – putting stress on your cells and organs, disrupting cell repair and altering liver and kidney function. You might suffer from headaches and sore muscles. Disruption to your digestion can lead to nausea, diarrhoea or constipation. Your mind is affected too, with brain fog making it hard to concentrate and easy to forget things.  All of this might be accompanied by a sense of irritability or apathy.

So what can you do to keep the worst effects of this jet set lifestyle at bay?

The chief culprit is not a lack of sleep, but disruption to the body's biological clock. This clock runs on a cycle of roughly 24 hours, urging us to eat, sleep and be active at the right times.

But every day the body clock needs resetting slightly because it is set to run at just over 24 hours. So, to keep us in sync, photosensitive retinal ganglion cells in the eye detect that it's getting dark. Then they send a cascade of messages, which results in the pineal gland – an organ the shape of a pine cone, which sits at the back of the brain – secreting the sleep hormone melatonin.

This system is run by a region of the brain called the supra-chiasmatic nuclei or SCN. Russell Foster, a professor of Circadian Neuroscience at the University of Oxford and the author of the book Life Time, explains that the SCN is like the conductor of an orchestra. It allows us to live our lives in neat cycles of night and day, he says.   

Or at least, that's what the SCN does until we take a long-haul flight. Then it's as though the orchestra has lost its conductor and things go awry.

Some of us are affected more than others, depending on factors such as our genetic make-up and regular medications. Symptoms tend to be worse the more time zones we cross, when the days are short and when flying east. The symptoms of jetlag are unpleasant, but for most of us they are short-term and occasional. However, there is a group of people for whom jet lag is unavoidable –airline crews. The precise pattern of an individual pilot or air steward's schedule can make a significant difference to their body's ability to recover. 

One study compared female flight attendants working on rotas that gave them little rest between flights, with flight attendants with more forgiving schedules who crossed fewer time zones and travelled less frequently. The staff in the group with the more demanding schedule were found to have higher levels of the stress hormone cortisol in their saliva.

This is fine in the short term, and we need cortisol to cope with the strain of waking up in the morning and to help us through stressful moments. But long-term exposure to high levels of cortisol can suppress the immune system and raise the risk of cardiovascular disease.

The impact of jet lag on the brain goes beyond just hormones. In a rather gruelling study conducted at the University of California Berkley in the US, hamsters were given a four-week sleep schedule which was the equivalent of them flying from New York to Paris twice a week. When the hamsters were "jet-lagged", they struggled with tasks they usually found easy.

What really surprised the researchers was how long the effects lasted – a month later the hamsters still weren't back on top form. Neuroscientific investigations revealed that while jet-lagged the hamsters had stopped creating as many new brain cells as usual in the hippocampus, a region of the brain associated with memory.

This fits with another finding from the flight attendant study, which also discovered changes in the brain. Scans showed that the group with the busy schedule had smaller right temporal lobes on average, compared to than the other group. Perhaps it wasn't surprising that busy individuals also obtained lower scores on tests of memory.

But the bad news for people who are frequently jet-lagged doesn't end there. Laboratory mice with induced jet lag show faster tumour growth. Foster speculates that if our circadian system is disrupted, it might stop us fighting off tumours at an early stage. On a more positive note, when a normal circadian rhythm was restored in the mice, tumour development slowed down.

Meanwhile in the lab chronic jet lag has been found to lead to a sequence of disease processes in mice that begins with non-alcoholic fatty liver disease, before progressing to hepatitis and a build-up of scar tissue in the liver, followed by a type of liver cancer.  

Of course, these studies were conducted in mice, not people – so the specific findings may not apply to us. But there is a higher incidence of various types of cancer in airline staff who frequently cross multiple time zones.

In a study that became well-known in the 1990s, Canadian pilots were found to have higher than average rates of prostate cancer. More recently, a meta-analysis of several studies found that female flight attendants have higher than average rates of breast cancer and melanoma. Research in which flight attendants were asked about their health found the chances of having cancer (which could be of any kind), were higher than in the general population in women, but not in men.

However, these studies are observational, so, we can't be certain that it's chronic jet lag which is causing the increased cancer risk. Precise causes are hard to disentangle from other factors such as exposure to cosmic radiation while flying. If jet lag is to blame, the study of flight attendants' brains which saw differences depending on the staff's rotates, could suggest that careful scheduling and adequate rest times could mitigate these some of the risks for airline staff.

Less frequent air travel has not been shown to bring these risks. But there are some things you can do to protect yourself from unpleasant symptoms, if you are crossing many time zones.

You can start before you even leave, by tilting your body clock in the right direction. If you are flying east, then go to bed one to two hours earlier than usual for a few days before your flight. If you are flying west, do the opposite, going to bed one to two hours later than usual.

Some people use the medication melatonin. In theory, it helps to reset your body clock by indicating that it will soon be night-time. When the Cochrane Review examined all the best evidence in 2002 it concluded that melatonin can be useful in combatting jet lag, but that the pharmacology and toxicology of melatonin needs more systematic study.

Whether or not health authorities recommend it depends on where you live. The Centers for Disease Control in the US includes advice on using melatonin for jet lag in their Yellow Book of official recommendations for travellers. In the UK, the NHS does not recommend it due to there not being enough evidence for its effectiveness.

Melatonin does bring risks. In the short-term, this medication can cause dizziness and nausea, while longer term daily use has been linked to a higher risk of heart failure, as well as an increased risk of hospitalisation from heart failure and increased risk of death overall. Anyone considering taking a new medication should consult their doctor first.

For short-term use of melatonin for jet lag, if you time the doses incorrectly, it can backfire. You can confuse your body clock entirely and make your jet lag worse. Online jet lag calculators can be useful here, but even then, some people dislike the feelings of drowsiness melatonin can cause, or can't take it due to health conditions such as epilepsy or due to interactions with other medication.

A safer approach, which can be used in combination with other strategies or alone, is to control your exposure to light once you arrive at your destination to delay or advance your body clock. But beware the simple advice to spend time outside. Again, you need to seek light at the right time, so at some specific times you may need to wear sunglasses in order to keep light out.

As an example, if you are travelling west, departing in the morning and crossing five time zones, you need to spend time outside on arrival. The light will tell your brain that it's still daytime, suppressing the release of melatonin and other signals for sleep. You need to stay awake until bedtime at your destination, however tempting it is to lie down on the bed.

Another option is to combine your light seeking with some exercise, which small studies have found can help shift the body clock. The advice to professional athletes trying to deal with jet lag is to continue training while jet lagged, but at low to moderate intensity.

• How airline pilots beat jet lag

• How to sleep like an Olympic athlete (should)

• Do you need less sleep as you age?

Your main aim is to get your body clock in sync with your new destination as soon as possible. Some people swear by changing their watch to their new time zone as soon as they get on a flight. And there is evidence that while you are en route, eating meals according to the time at your destination makes a difference too.

This isn't always easy to do on a plane because unless you're lucky enough to be in first class you don't get to choose what to eat and when. But eating at local mealtimes from arrival onwards still helps to shift the peripheral circadian clocks in the liver and the pancreas, helping to regulate your metabolism.

If that all sounds like a lot of work, there is one final study to bear in mind. Research from the University of Surrey which has not yet been peer-reviewed or published, asked infrequent long-haul travellers were asked how worried they were about getting jet lag. Those who were dreading it more, had worse symptoms, suggesting that the nocebo effect – where negative expectations lead to worse outcomes – may be at play.

So optimise your strategies, but try not to worry about it too much. Over time the body does adjust naturally to the time zone at your new destination, at a pace of about an hour a day if you've travelled east and an hour and a half a day if you've gone west. In a week or so's time you'll be feeling better anyway – and with only memories from your trip, and not ill effects.

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