Disruption to circadian rhythms caused by crossing time zones, which can result in broken sleep, with the sufferer waking during the night and then wanting to fall asleep during the day. The number of days this dysrhythmia lasts has been observed to be about equal to the number of time zones crossed. A study by Air New Zealand states that passengers crossing 12 times zones on a 26-hour flight require ten days to re-establish a normal sleep pattern (1).

The length of the flight is not the critical issue. The most important single factor is how many time zones you cross. People can suffer jet lag just crossing the United States (three hours' time change) but would be much less affected by a north-south flight of the same duration. The number of intermediate stops is also a factor, as each stop is accompanied by changes in cabin pressure. Lastly is your pre flight condition. If you are not fit, rested and healthy you will probably suffer more jet lag than others on the same flight.