|Classification and external resources|
|ICD-9-CM||307.45, 780.50 327.35|
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.
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, 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.
The condition is linked only to the trans-meridian (west–east or east-west) 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, one must subtract that number from 24. For example, 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.
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 most likely has the role of sending signals to peripheral oscillators which synchronize them for physiological functions. The SCN responds to light information sent from the retina. It is hypothesized 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 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 arises.
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||Local time to avoid light at destination||Local time to seek light at destination|
Traveling by 10 hours or more is usually best managed by assuming it is a 14h westward transition and delaying the body clock. A customized jet lag program can be obtained from an online jet lag calculator. These programs consider the sleep pattern of the user, as well as the number of time zones crossed and direction of travel (east or west). The efficacy of these jet lag calculators has not been documented.
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
Light therapy is a popular method used by professional athletes to reduce jet lag. Special glasses, usually battery-driven, provide light to the eyes, thus inhibiting the production in the brain of the hormone melatonin. Timed correctly, the light may contribute to an advance or delay of the circadian phase to that which will be needed at the destination. The glasses may be used on the plane or even before users leave their departure city.
The benefit of using melatonin is likely to be greater for eastward flights than for westward ones. There remain issues regarding the appropriate timing of melatonin use in addition to the legality of the substance in certain countries. How effective it may actually be is also questionable. For athletes, anti-doping agencies may prohibit or limit its use.
Timing of exercise and food consumption have also been suggested as remedies, 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 in how quickly they are able to 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 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 public.
For time changes of fewer than three hours, jet lag is unlikely to be a concern, and if travel is for short periods (three days or fewer) 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. However, crossing of the Arctic Ocean or even the North Pole (often the shortest route between N.E. Europe and Alaska or the Canadian West Coast and East Asia) does cause a significant time change. The jet travel from Alaska to N.E. Europe causes a pattern of a jet lag very similar to an eastward flight at lower latitudes.
In general, 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.
The Role of Melatonin
Melatonin follows a circadian pattern that is inversely related to levels of cortisol. This rhythm is disrupted in when travelling from west to east according to Wetterberg et al. A study conducted at Heathrow Airport that admitted patients to the psychiatric hospital found with 48% ill at arrival. 70% of the admitted patients with affective illnesses had a previous episode. Their finding support that predisposed individuals were more likely to be affected by time zone changes. There is strong support that jet lag is connected with relapse of affective disorders, even when demographic background is controlled for (such as religion). A proposed mechanism is that jet lag disrupts social rhythm which could be an onset of affective disorders, specifically mania. Timed melatonin administration has strong support from evidence and can be effective in reducing jet lag symptoms. Other methods involve hypnotic medication, stimulating substances (such as caffeine), and timed light exposure. Time light exposure can be effective to help people match their circadian rhythms with the expected cycle at their destination but requires strict adherence to timing and motivation. Melatonin could potentially be used to treat jet lag by "retraining the endogenous circadian rhythm”. Lengthening circadian rhythm is easier than shortening it; hence eastward travel may be more difficult than westward travel.
Mental Health Implications
Jet lag affects vulnerable individuals where travel could “precipitate acute mental illness” which may result from experiencing affective disturbances. When travelling across time zones, there is a “‘phase-shift’ of body temperature, rapid-eye-movement sleep, melatonin production, and other circadian rhythms”. Travelers travelling west are more likely to report depressive symptoms. Travelling west results in phase-advancement of the sleep-wake cycle in the traveller relative to their destination. Travellers travelling east are more likely to have manic-like symptoms; travelers are phase delayed in their sleep-wake cycle when compared to the sleep-wake cycle at their destination. Traveling east which was associated with manic-like symptoms may be strongly related to phase-delay. There has been evidence that sleep deprivation, which has phase delay as a consequence, has been effective for depression relief by improving mood. Short and intense sleep deprivation for one night has shown to improve mood in 66% of patients. The general standard procedure involves depriving the patient for about forty hours of sleep at night which continues into the next day. A proposed process for how sleep deprivation affects mood is that someone with depression generally has low S (sleep) process with their C (circadian rhythm) process unaffected; with sleep deprivation, the S process is brought to normal levels. However, when the individual resumes a regular sleep schedule, the S process resumes to a lower level. Therefore, sleep deprivation is effective in the short term but not in the long term for depression. Phase advancement is riskier for suicide than phase-delay. This disturbance in mood may be due to de-synchronization between the different and independent biological rhythms. This phenomenon may, in addition, be related to how travelling west is associated with depression mediated by phase advancement.
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