Jet lag, also jetlag or jet-lag, is a physiological condition which is a consequence of alterations to the circadian rhythm. Such disturbances result from shift work, daylight saving time, altered day length, or as the name implies, transmeridian travel as on a jet plane. They are known as desynchronosis, dysrhythmia, dyschrony, jet lag, or jet syndrome. The condition is generally believed to be the result of disruption of the "light/dark" cycle that entrains the body's circadian rhythm. It can be exacerbated by environmental factors.

The condition of jet lag may last many days, and medical experts have deemed that a recovery rate of "one day per time zone" is a fair guideline. Good sleep hygiene promotes rapid recovery from jet lag: in fact sleep, relaxation, moderate exercise, and sensible diet seem to be the simplest recovery methods.

However, since the experience of jet lag varies among individuals, it is difficult to assess the efficacy of any single remedy. Age may also be a factor on the severity of jet lag, as younger people will suffer worse jet lag than older people would .

Women are more susceptible to jet lag than are men this is in part because estrogen is often vulnerable to jet lag-like conditions .

Most chemical and herbal remedies, including the hormone melatonin, have not been tested nor approved by official agencies such as the U.S. Food and Drug Administration.

A recent study in hamsters showed that sildenafil (also known as Viagra) aided in a 50% faster recovery from shifts comparable to eastward travel experienced by humans and was effective starting at low doses. However, this use has not been tested in humans and is considered an off-label use by the drug's manufacturers.