What causes jet lag? Your body's daily cycle of sleep and wakefullness is governed by an internal clock (also known as your circadian rhythm). This clock governs when you will sleep, when you should wake and contributes to appetite regulation and other bodily rhythms. Jet lag is when your circadian rhythm falls out of sync with your current time zone. This is mainly due to travelling across time zones but it can be experienced by shift-workers, particularly those who work many late nights in a row.
Every day your body relies on many different cues to regulate your circadian rhythm. When you travel across time-zones or perform repeated shift work, this can cause your body's normal cycle of being awake in the day and asleep at night to be disrupted. This mismatch between stimuli from the external world and your internal clock can also lead to many other jet lag symptoms. Falling asleep early in the evening and waking up in the middle of the night or sleeping in until the afternoon are the obvious signs of jet lag. Yet it is also likely that a disrupted circadian rythym can cause you to feel other effects such as malaise, headache, nausea, diarrhoea, constipation, appetite change, irritability or light sensitivity.
Your circadian rhythm also has a strange quirk. Various researchers have shown that a typical human clock is actually longer than 24 hours. Estimates range from 24 hours, 11 minutes to 25 hours (one Earth rotation is a little shorter than 24 hours at 23 hours, 56 minutes). Deprived of any external cues, people will fall into an extended cycle of sleep and wakefulness, slowly going to bed and waking up later each day. During a typical day, your body uses many different signals to stay in the 24 hour rhythm. Understanding these important signals can allow you to make small changes in your activities that will help re-set your clock and cure jet lag. Everybody is different so some factors may influence you differently to others. These factors include:
Times when you fall asleep, including naps
Exposure to light, in particular light from the sun, blue light from sources such as electronic devices, phones, TVs and modern energy saving light globes (which tend to have more blue light than the older incandescent style bulbs)
The times you use stimulants like caffeine
Exercise or physical activity
Mental activity and stimulation
Another external factor affecting travel-related jet lag is poor air cabin pressure and dry air during flights than can lead to less oxygen entering your brain, while your body dehydrates. Lower oxygen causes discomfort and reduces your ability to fall asleep. This can further disrupt your circadian rhythm even before you arrive at your destination.
When you cross time zones, your body gets out of sync with these stimuli. For example, it's light when your body expects it to be dark, you are eating when you are supposed to be sleeping, you are trying to sleep when your body is expecting to be awake. This is why jet lag feels different to a night when you get poor sleep. You may have slept poorly on that one night but everything else such as daylight hours and eating are still in sync. However, a change in time zones causes your body to become confused. It's not merely that you are tired from the trip: your body's current position in its circadian rhythm is completely out of kilter with what it experiences in the new time zone.
That also explains an important point: flight direction is far more important than travelling time. Flying six hours north or south in the same time zone will not have much impact on your circadian rhythm compared to travelling east or west for the same amount of time.
Now that you have an understanding of the external cues and the circadian rythms, how can you prevent jet lagorcure it?