Jet lag is a physiological condition, often occurring when humans travel from one time zone to another. It happens because of the time difference between two places. Some people suffer from jet lag more than other people do.
If the departure time (the original body clock) is not the same as the destination time, jet lag can occur. Traveling east is more likely to cause jet lag than traveling west. That is because a cycle of the human's biological time is usually more than 24 hours, and going east makes the day shorter, which is a more difficult adjustment.
Jet lag can have several symptoms. It usually takes the form of fatigue and insomnia. Also, headache, depression, even nausea may strike people. Sufferers can use caffeine or sleeping pills as a way of treatment. Using caffeine (such as in tea or coffee) is helpful, either in the morning when going westward or in the afternoon when going eastward. The hormone melatonin is said to help people sleep. Also, doing things like changing sleeping patterns several days before departure may help people adjust to the new time zone.