Jet lag refers to a combination of symptoms that occur as the result of travel through three or more time zones. Everyone has an internal clock that is set to correspond with the external changes experienced in a 24-hour period; primarily relating to the day/night cycle. Traveling through several time zones can disrupt the body's internal clock; your body may think it's time to sleep even though it may be the start of a new day.
There are factors associated with travel that can contribute to jet lag. Traveling can be a stressful and tiring activity, causing fatigue by the time you reach your destination. Just being in a new environment can produce a feeling of disorientation. Many people find it difficult to sleep in a strange bed, and it may take a day or two to feel rested. The direction of travel can also influence the amount of jet lag experienced. Jet lag is more severe when traveling east than when flying west. It's easier for your body to extend the day rather than shorten it. Jet lag can last for several days.