A disruption to your circadian rhythms
Jet lag can occur anytime you cross two or more time zones. Jet lag occurs because crossing multiple time zones puts your internal clock or circadian rhythms, which regulate your sleep-wake cycle, out of sync with the time in your new locale.
For example, you lose six hours on a usual New York to Paris flight. That means that if you leave New York at 4:00 p.m. on Tuesday, you arrive in Paris at 7:00 a.m. Wednesday. According to your internal clock, it's 1:00 a.m., and you're ready for bed, just as Parisians are waking up.
And because it takes a few days for your body to adjust, your sleep-wake cycle, along with most other body functions, such as hunger and bowel habits, remains out of step with the rest of Paris.
The influence of sunlight
A key influence on your internal clock is sunlight. That's because light influences the regulation of melatonin, which in turn is a synchronizing signal to cells throughout the body.
Certain cells in the tissue at the back of your eye (retina) transmit the signal of light to an area of your brain called the hypothalamus. At night, when this signal is low, the hypothalamus tells the pineal gland, a small organ situated in the brain, to release the hormone melatonin. During the day, the light signals to the hypothalamus result in the opposite, such that the pineal gland produces very little melatonin.
You may be able to ease your adjustment to your new time zone by exposing yourself to daylight in the new time zone so long as the timing of light is done properly.
Airline cabin pressure and atmosphere
Some research shows that changes in cabin pressure and high altitudes associated with air travel may contribute to some symptoms of jet lag, regardless of travel across time zones.
Not drinking enough water during your flight can dehydrate you. In addition, drinking too many beverages with caffeine or alcohol during your flight can affect your sleep and cause some symptoms of jet lag.