Jet lag is a physiological condition that results from alterations to the body's circadian rhythms caused by rapid long-distance trans-meridian (east–west or west–east) travel. 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 timezone 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 is fully adjusted to the new time zone; a recovery period of one day per time zone crossed is a suggested guideline. 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. Travel by propeller-driven aircraft, by ship, or by train was slower and of more limited distance than jet flights, and thus did not contribute widely to the issue.