Wikipedia has defined Jet lag as under:

"Jet Lag (or "jetlag"), also jet syndrome, is a physical condition caused by crossing time zones during flight. The condition is generally believed to be the result of disruption to the circadian rhythms (i.e. the "light/dark" cycle) of the body. It can also be exacerbated by experiencing sudden changes in climate or seasonal conditions, as well as the reduced oxygen, partial pressure, excess noise and low humidity commonly experienced in the cabin of an aircraft"

Jetlag can cause many symptoms during air travel such as fatigue, constipation, headache, irregular heartbeat, and nausea. Hereunder are given tips to handle this syndrome.

• Have nice and sufficient sleep before boarding the plane.
• Take well-balanced meal.
• Avoid overeating don't drink alcohol as it dehydrates your body rather shift to fruits on the plane as intake of fruits re-hydrates the body
• Set your watch to the local time of the destination place on boarding the plane.
• Adjust your sleep time and eating times to the new location as soon as possible.
• Drink a lot of water during flight to avoid the dehydration as flight causes de-hydration.
• Use melatonin. It is a widely available over-the-counter at the airports. It helps some travelers re-adjust to the new location without experiencing jetlag.
• Do some exercise after small intervals during your journey. It will relax your body.