Jet lag occurs when the body has to adapt to a new light-dark schedule and normal daily routine after crossing several time zones.
Symptoms such as sleep disruption and tiredness are the result of your body finding it difficult to adjust to your new location's time zone because your body clocks can't adjust immediately.
Circadian rhythms are 24-hour variations in body functions, which are controlled by "biological clocks" in your body.
Jet lag occurs when your body's circadian rhythm needs to realign itself with a different light-dark schedule and daily routine.
The body's cells have their own clocks which interact with each other and are controlled by a "master" 24-hour clock in the brain.
The master clock helps to keep all the body clocks synchronised with each other and with the light-dark rest-activity schedule.
Your body is used to a regular routine of light and darkness at certain times of the day. But 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. After travelling so quickly, your body has to catch up and re-establish its circadian rhythm.
It takes time for your body to adjust the circadian rhythms to new times of light, darkness and eating.
East and west
Symptoms of jet lag are usually more severe when travelling east. This is possibly 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're extending your day rather than shortening it when you travel east.
It's usually easier to delay sleep for a few hours than trying to force sleep when you're not ready to.
General travel fatigue
Factors that can contribute to general travel fatigue (which isn't the same as jet lag) include:
- dehydration (not drinking enough water)
- drinking alcohol and caffeine during the flight
- lack of sleep
- being over 60 years of age
It's also thought high altitude and changes in cabin pressure may cause some symptoms similar to those of jet lag, regardless of travel across time zones.
Although the above factors can contribute to general travel fatigue, there's no evidence to suggest that they can make the symptoms of jet lag worse.