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Jet lag, medically referred to as desynchronosis and rarely as circadian dysrhythmia, is a physiological condition which results from alterations to the body's circadian rhythms resulting from rapid long-distance transmeridian (east–west or west–east) travel on high-speed aircraft. For example, someone travelling from New York to California feels as if the time were three hours later than local time. Jet lag was previously classified as one of the circadian rhythm sleep disorders. The condition of jet lag may last several days until one is fully adjusted to the new time zone, and a recovery rate of one day per time zone crossed is a suggested guideline. The issue of jet lag is especially pronounced for airline pilots, crew, and frequent travellers. Airlines have regulations aimed at combating pilot fatigue caused by jet lag. The common term "jet lag" is used, because before the arrival of passenger jet aircraft, it was generally uncommon to travel far and fast enough to cause jet lag. Trips in propeller-driven aircraft and trains were slower and of more limited distance than jet flights, and thus did not contribute widely to the problem. ==Cause== Jet lag is a chronobiological problem, similar to issues often induced by shift work and the circadian rhythm sleep disorders. When travelling across a number of time zones, the body clock (circadian rhythm) will be out of synchronisation with the destination time, as it experiences daylight and darkness contrary to the rhythms to which it has grown accustomed. The body's natural pattern is upset, as the rhythms that dictate times for eating, sleeping, hormone regulation, body temperature variations and other functions no longer correspond to the environment nor to each other in some cases. To the degree that the body cannot immediately realign these rhythms, it is jet lagged. The speed at which the body adjusts to the new schedule depends on the individual as well as the direction of travel; some people may require several days to adjust to a new time zone, while others experience little disruption. Crossing the International Date Line does not contribute to jet lag, as the guide for calculating jet lag is the number of time zones crossed, and the maximum possible disruption is plus or minus 12 hours. If the time difference between two locations is greater than 12 hours, one must subtract that number from 24. For example, the time zone GMT+14 will be at the same time of day as GMT−10, though the former is one day ahead of the latter. The condition is linked only to the trans-meridian (west–east or east-west) distance travelled. A ten-hour flight between Europe and southern Africa does not cause jet lag, as travel is primarily north–south. A five-hour flight between the Pacific and Atlantic coasts of the United States may well result in jet lag. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Jet lag」の詳細全文を読む スポンサード リンク
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