Travel by aircraft through several time zones leads to desynchronization of internal bodily rhythms and a phase shift between physiological and psychological functions. The problem starts when our normal day-to-night schedule changes suddenly, along with established patterns of eating, sleeping and other activities. The sudden shift in the arrival of day and night "confuses" the mechanisms that regulate the rhythms of daily functioning, particularly in relation to the sleep-wake cycle. Adding to the problem are the physical results of prolonged sitting and hours of exposure to dry, recycled air.
Most travellers become more jet-lagged flying east than they do flyng west. Flying east, the day shortens; it is hard to stay asleep, and one is tired in the morning. Flying west, the day lengthens, so one wakes too early and is sleepy in the evenings.
The disorientation creates a special difficulty for international diplomatic and business missions since the travelling party is at a disadvantage in any meeting following arrival. There is also some evidence that suggests the existence of cumulative long-term effects which shorten the life span in the case of frequent travel of this kind.
Jet lag, which is partly sleep deprivation and partly bodily confusion, causes efficiency problems for transnational corporations and military organizations which need to be assured that their representatives (businessmen, soldiers) are fully alert after a long journey across time zones. Airline pilots spending a week or longer on international flight schedules with long hours and multiple crossings of time zones have difficulty staying awake while flying and sleeping during layovers. For holiday travellers, the reorientation period inconveniently cuts into their allotted vacation time.
Circadian rhythms are biological rhythms which recur on a roughly 24-hour basis. The rhythms include sleeping patterns, hormonal release and temperature cycles.
Jet lag has become a real and persistent problem in industrialized societies. Neurotics seem to be particularly sensitive to this temporary dislocation. A frequently cited example is the failure of USA negotiations with Egypt in 1950, because of the jet lag of the Secretary of State, resulting in the contract for the Aswan Dam being given to the USSR, thus providing the USSR with its first base of operations in Africa.