After flying east or west over several time zones, most people do not overcome the “jet lag” for several days. Out of step with their environment, they feel the urge to eat and sleep on the schedule they followed at home. Since traveling the same distance north and south involves no time zone change, there is no jet lag.
This is not only because of a difference in the person’s sleeping pattern, but also because it changes the time of day when the hormones in the body, such as adrenalin and thyroid, are most plentiful. Hormone production cannot change suddenly and can only adjust by about one hour a day, as we pointed out in the article “Shift Work and Hormones” under the heading Hormones. Since hormone blood levels can’t peak in synchrony with the changed hours of the traveler, the person experiences jet lag.
Surprisingly, more time is needed to overcome jet lag after an eastward journey than after an equally long journey westward. This “directional asymmetry,” according to the British Medical Journal (284:144), is due to the fact that traveling westwards results in a longer day than usual during the journey, whereas traveling eastwards shortens the day. The human body, research has shown, prefers a long day to a short one. People living without clocks and in a building without windows naturally adopt a sleep-wake cycle of 25 hours or longer in preference to the 24-hour day.
*165\143\2*
