# Jet lag

Jet lag is a temporary physiological condition that occurs when a person's circadian rhythm is out of sync with the time zone they are in, and is a typical result from travelling rapidly across multiple time zones (east–west or west–east). For example, someone travelling 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. The phase shift when travelling from east to west is referred to as phase-delay of the circadian cycle, whereas going west to east is phase-advance of the cycle. Most travellers find that it is harder to adjust time zones when travelling east. Jet lag was previously classified as a circadian rhythm sleep disorder.
The condition may last several days before a traveller becomes fully adjusted to a new time zone; it takes on average one day per time zone crossed to reach circadian reentrainment. 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 was created after the arrival of jet aircraft, because prior that it was uncommon to travel far and fast enough to cause the condition.

## Discovery

According to a 1969 study by the Federal Aviation Administration, aviator Wiley Post was the first to write about the effects of flying across time zones in his 1931 co-authored book, Around the World in Eight Days.

## Signs and symptoms

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 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 and consistency of bowel movements, and reduced appetite. 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 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. This is often a common concern at international sporting events like the Olympics and FIFA World Cup. However many athletes arrive at least 2–4 weeks ahead of these events, to help adjust from any jet lag issues.

### 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 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.

## Cause

Jet lag is a chronobiological problem, similar to issues often induced by shift work and circadian rhythm sleep disorders. When travelling across a number of time zones, a person's 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 was accustomed. The body's natural pattern is disturbed, as the rhythms that dictate times for eating, sleeping, hormone regulation, body temperature variation, 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 a new rhythm 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 absolute time difference between two locations is greater than 12 hours, one must subtract 24 from or add 24 to that number. 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 distance travelled along the east–west axis. 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.

### Double desynchronisation

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 peripheral oscillators in tissues. The SCN's role is to send signals to the 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

Delayed sleep phase disorder is a medical disorder characterised by delayed sleeping time and a proportionately delayed waking time due to a phase delay in the internal biological master clock. Specific genotypes underlie this disorder. If allowed to sleep as dictated by their endogenous clock these individuals will not have any ill effects as a result of their phase-shifted sleeping time.

## Management
### Light exposure

Light is the strongest stimulus, or zeitgeber, for realigning a person's circadian cycle, and the key to quick adaptation is therefore timed light exposure based on the traveller's sleep pattern, chronotype, and plans.
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. Timed correctly, the light may contribute to an advance or delay of the circadian phase to match the destination.
The US Centers for Disease Control and Prevention (CDC) recommends mobile apps for the correct timing of light exposure and avoidance, when to use caffeine, and when to sleep.

### Melatonin administration

In addition to timed light exposure, the right type and dose of melatonin, at the right time, can help travellers shift faster and sleep better as they are transitioning between time zones. There are issues regarding the appropriate timing of melatonin use, in addition to the legality of the substance in certain countries. 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.

### Short duration trips

In the case of short duration trips, jet lag may be minimised by maintaining a sleep-wake schedule based on the originating time zone after arriving at the destination, but this strategy is often impractical in regard to desired social activities or work obligations. Shifting one's 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 further reduced through a combination of artificial exposure to light and rescheduling, as these have been shown to augment phase-shifting.

### Pharmacotherapy

The short-term use of hypnotic medication has shown efficacy in reducing insomnia related to jet lag. In a study, zolpidem improved sleep quality and reduced awakenings for people travelling across five to nine time zones. The potential adverse effects of hypnotic agents, like amnesia and confusion, have led some doctors to advise patients to test such medications prior to using them for treating jet lag. Several cases using triazolam to promote sleep during a flight reported dramatic global amnesia.

## Mental health implications

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 study found that relapse of bipolar 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 circadian rhythm disruption has 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.