|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 high-speed aircraft. 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.
Jet lag is a chronobiological problem, similar to issues often induced by shift work and the circadian rhythm sleep disorders. When travelling 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 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 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 careful control of exposure to and avoidance of bright lights can speed adjustment to a new time zone.
Management after travelling east
Traveling east causes more problems than traveling west, because the body clock has to be advanced, which is harder than delaying it, and the necessary exposure to light to realign the body clock does not tie in with the day/night cycle at the destination.
Traveling east by six to nine time zones causes the biggest problems, as it is desirable to avoid light in the mornings.
Waterhouse et al.  recommend:
|Time zones||Time to avoid light||Time to seek light|
Traveling by 10 hours or more is usually best managed by assuming it is a 14h westward transition and delaying the body clock.
Management when travelling west
Travelling west causes fewer problems than travelling east, and it is usually sufficient to seek exposure to light during the day and avoid it at night.
Other management methods
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 there are questions regarding how effective it may actually be. For athletes, anti-doping agencies may prohibit or limit its use. Timing of exercise and food consumption have also been suggested, though their applicability in humans and practicality for most travellers are not certain and no firm guidelines exist. There is very little data supporting the use of diet to adjust to jet lag. While there are data supporting the use of exercise, the intensity of exercise that may be required is significant, and possibly difficult to maintain for non-athletes. These strategies may be used both before departure and after landing. Individuals may 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 within 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. A westward adjustment takes, in days, approximately half the number of time zones crossed. For eastward travel, adjusting to the new time zone takes, in days, approximately two-thirds the number of time zones crossed.
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- "Highlights of Changes from DSM-IV-TR to DSM-5" (PDF). American Psychiatric Association. May 17, 2013. Retrieved May 23, 2013.
- Waterhouse, J. (1999). "Jet-lag and shift work: (1). Circadian rhythms". Journal of the Royal Society of Medicine 92 (8): 398–401. PMC 1297314. PMID 10656004.
- Waterhouse, J.; Reilly, T.; Atkinson, G.; Edwards, B. (2007). "Jet lag: Trends and coping strategies" (pdf). The Lancet 369 (9567): 1117–1129. doi:10.1016/S0140-6736(07)60529-7. PMID 17398311.
- Waterhouse, J.; Edwards, B.; Nevill, A.; Carvalho, S.; Atkinson, G.; Buckley, P.; Reilly, T.; Godfrey, R.; Ramsay, R. (2002). "Identifying some determinants of "jet lag" and its symptoms: A study of athletes and other travellers". British journal of sports medicine 36 (1): 54–60. doi:10.1136/bjsm.36.1.54. PMC 1724441. PMID 11867494.
- Forbes-Robertson, S.; Dudley, E.; Vadgama, P.; Cook, C.; Drawer, S.; Kilduff, L. (2012). "Circadian Disruption and Remedial Interventions". Sports Medicine 42 (3): 185–208. doi:10.2165/11596850-000000000-00000. PMID 22299812.