|Classification and external resources|
|ICD-9||307.45, 780.50 327.35|
Jet lag, medically referred to as desynchronosis, 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 a (typically jet) aircraft. It is 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 the passenger jet aircraft, it was generally uncommon to travel far and fast enough to cause jet lag. Propeller flights were slower and of more limited distance than jet flights, and thus did not contribute as widely to the problem.
Jet lag is a chronobiological problem, similar to issues often induced by shift work. When traveling across a number of time zones, the body clock 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 and body temperature variations 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; some people may require several days to adjust to a new time zone, while others experience little disruption. Crossing one or two time zones does not typically cause jet lag.
The condition is not linked to the length of flight, but to the trans-meridian (west–east) distance traveled. A ten-hour flight from Europe to southern Africa does not cause jet lag, as travel is primarily north–south. A five-hour flight from the east to the west coast of the United States may well result in jet lag.
Crossing the International Date Line does not contribute to jet lag, as the guide for calculating jet lag is the number of time zones crossed, and the maximum possible disruption is plus or minus 12 hours. If the time difference between two locations is greater than 12 hours, subtract that number from 24. Note, for example, that the time zone GMT+14 will be at the same time of day as GMT−10, though the former is one day ahead of the latter.
The symptoms of jet lag can be quite varied, depending on the amount of time zone alteration, time of day and the susceptibility of individual differences. Sleep disturbance occurs, with poor sleep upon arrival, sleep disruption including trouble falling asleep (if flying east), early awakening (if flying west) and interrupted sleep with multiple awakenings 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 feces and reduced interest in and enjoyment of food. Symptoms are caused by a circadian rhythm that is out of sync with the day-night cycle of the destination. 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 usually requires 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 of daylight saving time. Symptoms and consequences of jet lag can be a significant area of 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 
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 limited food and dry air. It does not necessarily have 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 high-quality sleep.
Light is the strongest stimulus for re-aligning a person's sleep-wake schedule and the careful control over exposure and avoidance of bright lights can speed adjustment to a new time zone. Melatonin is used to adjust the circadian clock but there are issues regarding the appropriate dosage and dosage timing, in addition to the legality of the substance in certain countries. In addition, its effects may not carry-over beyond its immediate use, and there are questions regarding how effective it may actually be. There are also concerns regarding its use by people with the symptoms of severe mental illness, severe allergies, autoimmune diseases, immune system cancers, or pregnant women. For athletes, anti-doping agencies may also prohibit or limit its use. Timing of exercise and food consumption have also been suggested, though their applicability in humans and practicality for most travelers are not yet certain and no firm guidelines exist. There is very little data supporting the use of diet to adjust to jet lag. While there is data supporting the use of exercise, it is hard to determine if the exercise is responsible, or the accompanying exposure to sunlight, and the intensity of exercise that may be required is significant, and possibly difficult to maintain for non-athletes. Use of these strategies can occur both before departure and after landing. Individuals may also differ in their susceptibility to jet lag and ability to quickly adjust to new sleep-wake schedules.
Short-acting sleep medications can be used to improve sleep quality and timing, and stimulants can be used to promote wakefulness, though both these interventions are not generally used in non-military situations and research results on their success at adapting to jet lag are inconsistent. Among the stimulants, only caffeine may be readily available to the general public.
For time changes of less than three hours, jet lag is unlikely to be a concern, and if travel is for short periods (three days or less) retaining a "home schedule" may be better for most people. Sleeping on the plane is only advised if it is the destination's normal sleep time.
Direction of travel 
North-south flights that do not cross time zones do not cause jet lag. Adjustment to the new time zone is easier for east-to-west travel than west-to-east. For eastward travel, adjusting to the new time zone takes, in days, approximately two-thirds the number of time zones crossed. A westward adjustment takes, in days, approximately half the number of time zones crossed.
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