You get off the plane bleary eyed and yawning, but the excitement of being in a new country keeps you on the go.

It's almost enough to make you forget that just hours ago you were cramped up and trying to sit-sleep in a large metal tube, being fed meals at odd times, all while flying at 900 kilometres per hour.

Finally, it overwhelms you. Heavy-limbed, you surrender — the accumulation of fatigue not only from crossing time zones, but also the exhaustion from travelling far from home.

If you feel out of sync, it makes sense.

What is jet lag?

There are three parts to understanding jet lag, Sleep Health Foundation chair Professor Dorothy Bruck says.

Firstly, there's the time difference, as your body is required to go to sleep and wake up at times it's not used to.

Secondly, there's the actual fatigue from travelling. It's easy to forget that even before the plane takes off, there's the last-minute packing affecting your sleep the previous night, the commute to the airport and the many procedures at the airport.

"The tiredness that you feel just from having travelled a long way, independent of the changing time difference, is a big part of jet lag," says Professor Bruck.

Thirdly, your body rhythms. Their regular beat takes time to adjust to the new time zone.

How does jet lag affect body rhythms?

"We don't just have one body rhythm, we have many body rhythms, and we have many clocks that guide the different rhythms," Professor Bruck says.

Across a 24-hour period, there are rhythms for when we feel sleepy and when we don't, rhythms for when we feel hungry or when we want to urinate, as well as lots of other internal rhythms of hormones and metabolisers.

The body clocks we commonly think about with jet lag are 24-hour clocks, but we also have clocks that run to shorter rhythms; for example, three- or four-hour rhythms.

There are brainwaves, which are very quick rhythms, and the heart rate which is yet another rhythm.

"Now when we, say, fly to London from Melbourne, those rhythms will realign at different rates because there are many different clocks that regulate all the different rhythms of our body," says Professor Bruck.

The time difference plus the change in your body rhythms is why it can feel so strange to do things like sleep and eat in your new time zone.

What you can do to deal with jet lag

Heading straight to bed might be what you feel like doing, but getting out and about in the sunshine is actually a better way to address jetlag. ( Unsplash: Emma Simpson )

The Sleep Health Foundation has the following recommendations for trying to keep the ill-effects of jet lag to a minimum:

For short trips of only a day or two, stay on home time

For short trips of only a day or two, stay on home time For longer trips of three days or more, change your time as soon as possible

For longer trips of three days or more, change your time as soon as possible Give yourself time. Adjusting to a new time zone usually takes two or three days

Give yourself time. Adjusting to a new time zone usually takes two or three days Take short naps, no longer than 30 minutes. And if you're napping, try to be awake for at least 4 hours before bedtime

Take short naps, no longer than 30 minutes. And if you're napping, try to be awake for at least 4 hours before bedtime Caffeine may be helpful, but don't overdo it. Don't have tea or coffee for at least two hours before bed

Caffeine may be helpful, but don't overdo it. Don't have tea or coffee for at least two hours before bed Alcohol is not the solution. Although it may help you to get off to sleep, you will not sleep as well during the night

Alcohol is not the solution. Although it may help you to get off to sleep, you will not sleep as well during the night Minimise the use of sedatives and sleeping tablets. They can become a habit, giving you more problems than temporary jet lag

Minimise the use of sedatives and sleeping tablets. They can become a habit, giving you more problems than temporary jet lag Go outside. Sunlight is important to help your body adjust to the new time zone

Go outside. Sunlight is important to help your body adjust to the new time zone Do some exercise. This will help reset your body clock, especially when done during daylight hours and not too close to bedtime.

Understanding the link between jet lag and melatonin

The hormone melatonin is produced in the pituitary gland of your brain and regulates when you feel sleepy.

"Melatonin is like a vampire," says Professor Bruck. "It hates light."

In the early evening, if you are in a stable time zone, melatonin levels start to rise but this rise becomes more noticeable in the later evening as you're getting ready for bed.

Melatonin levels will peak, usually at around 3:00am or 4:00am, and will then start to decline, right up to the moment you open the curtains when you wake up and get a lot of light.

"So the ideal thing we want is good solid melatonin rhythms at night when it's dark, and no melatonin during the day," says Professor Bruck.

Is melatonin the reason sun exposure is so often recommended?

Exactly. It's basically to stop the melatonin from rising at the time that it would when you're back home.

Say, for example, you're flying to London from Melbourne and there's a nine-hour time difference.

In order to have a good night's sleep that first night, you want to try to stop the melatonin rising at its regular Melbourne time. If you get some sun, it will increase the chance of your melatonin levels rising when it's early evening in London, and not earlier as it would in Melbourne.

"Also, if you've stayed awake all day, which is the ideal thing to do if you arrive in the early morning, your melatonin is just bursting to rise again, and also you're very tired, so you've got two things at play there to getting a good night's sleep," adds Professor Bruck.

It's worth noting that it takes some time for the body to adjust, and Professor Bruck notes that while the first and second nights are quite good, you often start to have problems and wake up more in the morning of your third day.

"That's because the clock is still is not running on London time, so it's not having enough of a good amplitude of melatonin all through the night to keep you asleep in the morning," she says.

Will cutting down on screen time help my jet lag?

If it's leading up to bedtime, it's best to cut down on screen time if you want to increase your chance of having a good night's sleep.

Melatonin is like a vampire, remember? It's the blue component of light that suppresses melatonin, says Professor Bruck, so if you've got your screen up without something that changes the blue background to an "orange-y" background, you'll actually be suppressing your melatonin at the time you need it the most.

How does jet lag affect my digestive system?

When you're travelling, you might want to take extra care with what and how much you're eating, because your digestive system is running on body clocks that aren't in sync with its new time zone.

The body clocks tell us when to secrete a whole lot of enzymes and various chemicals throughout the body.

For example, when you're hungry and can feel it in your stomach, that's driven by your body clock and all the gastric juices saying, "Give me food, I'm ready to break it down".

But when you cross into a new time zone, you're eating at times that may not be dictated by when your stomach is prepared for food. Professor Bruck compares this to shift workers who have trouble with their digestion.

"They'll eat when it suits the family or before they go on shift, and it's not necessarily when their tummy is ready with the gastric juices. It's all mucked up."

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How does exercise help jet lag?

The body clock responds to activity and does better when you're active in the daylight hours.

"It seems to help the rhythm work the way it's supposed to, which is why we recommend you do exercise to help with jet lag," says Professor Bruck.

And if you're able to exercise outdoors with sunlight, double bonus.

You're on your way to getting your body clocks closer to working in the time zone of your new city. Happy travels!