Jet lag is a physiological condition that results from alterations to the body's circadian rhythms caused by rapid long-distance trans-meridian (east-west or west-east) travel. For example, someone flying from New York to London, i.e. from west to east, feels as if the time were five hours earlier than local time, and someone travelling from London to New York, i.e. from east to west, feels as if the time were five hours later 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, aircraft 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 issue.
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; dizziness, nausea, insomnia, confusion, anxiety, 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 travelling east or west to competitions, as performance is often dependent on a combination of physical and mental characteristics that are affected 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 one 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 travelling across a number of time zones, the body clock (circadian rhythm) will be out of synchronisation 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 four-hour flight between Miami, Florida and Phoenix, Arizona in the United States may result in jet lag, as the direction of travel is primarily east-west.
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-synchronisation is exacerbated as the body is not in sync with the environment - a "double desynchronisation", which has implications for health and mood.
Delayed sleep phase disorder is a medical disorder characterized by delayed sleeping time and a proportionately delayed waking time due to a phase delay in the endogenous biological master clock. Specific genotypes underlie this disorder. If allowed to sleep as dictated by their endogenous clock these individuals do not suffer any ill effects as a result of their phase shifted sleeping time.
Light is the strongest stimulus for realigning a person's sleep-wake schedule, and careful control of exposure to and avoidance of bright light to the eyes can speed adjustment to a new time zone. The hormone melatonin is produced in dim light and darkness in humans, and it is eliminated by light.
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 northeast Europe and Alaska or the Canadian West Coast and East Asia) does cause a significant time change. Jet travel from Alaska to northeast Europe causes a pattern of jet lag very similar to an eastward flight at lower latitudes. Also seasonal differences in sunlight if one crosses the equator may make a slightly disrupted sleeping pattern at the destination.
In general, adjustment to the new time zone is faster for east-west travel than for west-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. Studies have shown that performance in both individual and team sports is measurably better in athletes who have flown westward to the venue than in the opposite direction.
Travelling east causes more problems than travelling west because the body clock has to be advanced, which is more difficult for the majority of humans than delaying it. Most people have an endogenous circadian rhythm that is longer than 24 hours, so lengthening a day is less troublesome than shortening it. Equally important, the necessary exposure to light to realign the body clock does not tie in with the day/night cycle at the destination.
Travelling 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|
Travelling east by 10 hours or more is usually best managed by assuming it is a 14-hour westward transition and delaying the body clock. A customised jet lag program can be obtained from an online jet lag calculator. These programs consider the sleep pattern of the user, the number of time zones crossed, and the direction of travel. The efficacy of these jet lag calculators has not been documented.
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.
Timed light exposure can be effective to help people match their circadian rhythms with the expected cycle at their destination; it requires strict adherence to timing. 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 of melatonin in the brain. 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.
Timed melatonin administration may be effective in reducing jet lag symptoms. The benefit of using melatonin is likely to be greater for eastward flights than for westward ones because for most people it is easier to delay than to advance the circadian rhythm. 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.
Melatonin can be considered to be a darkness signal, with effects on circadian timing that are the opposite of the effects of exposure to light. Melatonin receptors are situated on the suprachiasmatic nucleus, which is the anatomical site of the circadian clock. The results of a few field studies of melatonin administration, monitoring circadian phase, have provided evidence for a correlation between the reduction of jet lag symptoms and the accelerated realignment of the circadian clock.
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 is 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 can adjust to new sleep-wake schedules.
Short-acting sleep medications can be used to improve sleep quality and timing, and stimulating substances such as caffeine can be used to promote wakefulness, though research results on their success at adapting to jet lag are inconsistent.
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.
In the case of short duration trips, an easy way to minimize jet lag is to maintain the sleep-wake schedule from home after arriving at the destination, but this strategy is often unpractical in regard to desired social activities or work obligations . Shifting your sleep schedule before departure by 1-2 hours to match the destination time zone may also shorten the duration of jet lag . Symptoms can be reduced even more through a combination of artificial exposure to light and rescheduling, as it has been shown to augment phase-shifting.
A short hypnotic medication has been effective to reduce insomnia related to jet lag. In a study, zolpidem improved sleep quality and reduced awakenings for people traveling across five to nine time zones. Potential adverse effects of hypnotic agents, like amnesia and confusion, should be taken into account. Several cases using triazolam to promote sleep during a flight reported dramatic global amnesia.
Jet lag may affect the mental health of vulnerable individuals. When travelling across time zones, there is a "phase-shift of body temperature, rapid-eye-movement sleep, melatonin production, and other circadian rhythms". A 2002 Israeli study found that relapse of major affective and psychotic disorders occurred more frequently when seven or more time zones had been crossed in the past week than when three or fewer had been crossed. Although significant disruptions of circadian rhythms had been documented as affecting individuals with bipolar disorder, an Australian team studied suicide statistics from 1971 to 2001 to determine whether the one-hour shifts involved in daylight saving time had an effect. They found increased incidence of male suicide after the commencement of daylight saving time but not after returning to standard time.