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 traveling from New York to California feels as if the time were three hours later. It 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 travelers. 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 as widely to the problem.