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Jet lag - Causes of jet lag

NHS Choices Medical Reference

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Jet lag occurs when the body's

normal daily routine is disrupted after

crossing several different time zones.

Symptoms such as sleep disruption and tiredness are the result of your body finding it difficult to adjust to the time zone of your new location.

Circadian rhythm

Circadian rhythm is your body's natural 24-hour routine, which is controlled by "biological clocks" in your body. Jet lag occurs when your body's circadian rhythm is disrupted.

The biological clocks are found throughout your body and are made up of groups of cells that interact with each other. The cells are controlled by a "master clock" in your brain that keeps all the body clocks synchronised.

Your body is used to a regular routine of light and darkness at certain times of the day. However, when you travel to a new time zone this routine is disrupted.

Air travel makes it possible to cross several different time zones in just a few hours. Due to travelling so quickly, your body has to catch up and re-establish its circadian rhythm. It takes time for your body to adjust to new times of light, darkness and eating. It may also have to adjust to differences in temperature.

Oxygen levels

Oxygen levels in an aeroplane cabin are also thought to play a role in jet lag. The air pressure found in an aeroplane cabin is lower than normal, which means the amount of oxygen in your blood is reduced.

A reduced amount of oxygen can affect your physical and mental abilities. For example, it can make you feel lethargic (lacking in energy) and dehydrated (when the normal water content in your body is reduced).

If you have a health condition, such as heart disease, lung disease or anaemia (where red blood cells are unable to carry enough oxygen), you may be more severely affected by the reduction in oxygen inside the cabin.

East and west

Symptoms of jet lag are usually more severe when travelling east. This is because your body finds it easier to adapt to a  longer day (you "gain time" travelling west) than a shorter one (you "lose time" travelling east).

Your body is able to adapt better when you travel west because you are extending your day, rather than shortening it when you travel east. Therefore, it's usually easier to delay sleep for a few hours than trying to force sleep when you're not ready to.

Increased risk

Other things that increase jet lag or increase the severity of your symptoms include:

  • dehydration (not drinking enough fluids)
  • lack of sleep
  • drinking alcohol
  • stress 
  • being over the age of 60
Medical Review: May 13, 2012
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