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 trans-meridian (east–west or west–east) travel. For example, someone travelling from New York to London feels as if the time were five hours earlier 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 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, 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 problem.
The symptoms of jet lag can be quite varied, depending on the amount of time zone alteration, time of day, and individual differences. Sleep disturbance occurs, with poor sleep upon arrival and/or sleep disruptions such as trouble falling asleep (when flying east), early awakening (when flying west), and trouble remaining asleep. Cognitive effects include poorer performance on mental tasks and concentration; increased fatigue, headaches, and irritability; and problems with digestion, including indigestion, changes in the frequency of defecation and consistency of faeces, and reduced interest in and enjoyment of food. The symptoms are caused by a circadian rhythm that is out of sync with the day-night cycle of the destination, as well as the possibility of internal desynchronisation. Jet lag has been measured with simple analogue scales, but a study has shown that these are relatively blunt for assessing all the problems associated with jet lag. The Liverpool Jet Lag Questionnaire was developed to measure all the symptoms of jet lag at several times of day, and this dedicated measurement tool has been used to assess jet lag in athletes.
Jet lag may require a change of three time zones or more to occur, though some individuals can be affected by as little as a single time zone or the single-hour shift to or from daylight saving time. Symptoms and consequences of jet lag can be a significant concern for athletes traveling east or west to competitions, as performance is often dependent on a combination of physical and mental characteristics that are impacted by jet lag.
Travel fatigue is general fatigue, disorientation, and headache caused by a disruption in routine, time spent in a cramped space with little chance to move around, a low-oxygen environment, and dehydration caused by dry air and limited food and drink. It does not necessarily involve the shift in circadian rhythms that cause jet lag. Travel fatigue can occur without crossing time zones, and it often disappears after a single day accompanied by a night of good quality sleep.
Jet lag is a chronobiological problem, similar to issues often induced by shift work and the circadian rhythm sleep disorders. When traveling across a number of time zones, the body clock (circadian rhythm) will be out of synchronization 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 in itself contribute to jet lag, as the guide for calculating jet lag is the number of time zones crossed, with a maximum possible time difference of 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 UTC+14 will be at the same time of day as UTC−10, though the former is one day ahead of the latter.
Jet lag 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 the direction of 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.
There are two separate processes related to biological timing: circadian oscillators and homeostasis. The circadian system is located in the suprachiasmatic nucleus (SCN) in the hypothalamus of the brain. The other process is homeostatic sleep propensity, which is a function of the amount of time elapsed since the last adequate sleep episode.
The human body has a master clock in the SCN and also peripheral oscillators in tissues. The SCN's role is to send signals to peripheral oscillators, which synchronise them for physiological functions. The SCN responds to light information sent from the retina. It is hypothesised that peripheral oscillators respond to internal signals such as hormones, food intake, and "nervous stimuli"
The implication of independent internal clocks may explain some of the symptoms of jet lag. People who travel across several time zones can, within a few days, adapt their sleep-wake cycles with light from the environment. However, their skeletal muscles, liver, lungs and other organs will adapt at different rates.This internal biological de-synchronization is exacerbated as the body is not in sync with the environment—a "double desynchronization", which has implications for health and mood.