Jet lag

Jet lag is a temporary physiological condition that occurs when one's body's circadian rhythms is out of sync with the time zone that they're in, and is a typical result from doing rapid long-distance travel across multiple time zones (east–west or west–east trans-meridian). For example, someone flying from New York to London, i.e. from west to east, feels as if the time were five hours earlier than local time, and someone travelling from London to New York, i.e. from east to west, feels as if the time were five hours later than local time. The phase shift when traveling from east to west is referred to as phase-delay of the circadian circle, whereas going west to east is phase-advance of the circadian circle. Most travelers find that it is harder to time zone adjust when traveling to the east. Jet lag was previously classified as one of the circadian rhythm sleep disorders.

The condition of jet lag may last several days before the traveller becomes fully adjusted to the new time zone; and it takes on average, one day per time zone crossed, to reach circadian readaptation. Jet lag is especially an issue for airline pilots, aircraft crew, and frequent travellers. Airlines have regulations aimed at combating pilot fatigue caused by jet lag.

The term "jet lag" is used because before the arrival of passenger jet aircraft, it was uncommon to travel far and fast enough to cause desynchronosis.