Jet lag affects travelers flying east-west or west-east journey, crossing the time zone and finding themselves in a place where the local time differs from the home time. The rising and setting of the Sun is out of sync with their body clock, which is still on time due to 24-hour biological rhythms. So jet lag is the failure of the body to adjust its own routine to a clock that may, for instance, bring darkness (and bedtime) ten hours earlier or later than usual. In such conditions, the long-haul passengers feel like going to sleep just as darkness turns to light with the rising of the Sun, and just as everyone else is waking up ready to start the new day. Eating and exercising are also uncomfortably affected and mental reactions may slow down considerably. The result could be headaches, poor sleep, constipation, poor mental performance, giddiness and even slight depression.
The effects of jet lag seem to be greater on eastbound flights than on westbound. This is because traveling westwards you are traveling with the Sun, and the hours of daylight you experience will be longer than normal. The day will appear to be longer than 24 hours. This goes well with the biological clock, because many of the body rhythms have a period of 25 hours and they show a natural tendency to move towards the new time zone at the rate of about one hour a day. Conversely, when traveling eastwards, the day is shortened. So, body rhythms have to be pulled back to less than 24 hours, against their natural tendency to lengthen.
There is no way one can prevent jet lag and it is not possible to bring all the body rhythms back in sync immediately. Experience shows that one full day of natural recovery is needed for each five-hour time change. However, travelers should try to go to bed as near as possible to their usual bedtime on the first night after arriving. This helps, though not much.
Jet lag (Wikipedia)