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. 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 is caused by a misalignment between the internal circadian clock and the external environment, and it has been classified within the category of a circadian rhythm sleep disorder, reflecting its basis in disrupted biological timing rather than general travel fatigue.
The condition may last several days before a traveller becomes fully adjusted to a new time zone; it takes on average one day per hour of time zone change to reach circadian re-entrainment. 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.
Jet lag has been the subject of research across multiple fields including chronobiology, sleep medicine, and aviation health. Numerous peer-reviewed studies have examined its underlying mechanisms, health implications, and treatment strategies. Research efforts are ongoing, particularly within laboratories focused on circadian biology and sleep disorders, reflecting the condition's relevance to both clinical practice and occupational health.
The term jet lag was created after the arrival of jet aircraft, because prior to 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. However, Post and the Federal Aviation Administration initially did not link these symptoms to circadian disruption. Instead, these effects were attributed to travel fatigue factors such as anxiety about air travel, disruption to routine, and dehydration from the dryness of the prolonged aircraft time. Jet lag started associating with circadian rhythms in the 1970s with the rise of entrainment and phase shift research, mimicking similar symptoms to jet lag when rhythms were disrupted. These associations began the official recognition of jet lag as an effect of circadian disruption rather than a factor of travel fatigue.The term jet lag is inspired by how people feel after travelling rapidly over several time zones, typically on a plane or jet-like form of transportation. The body's feeling of having to adjust to the new time zone serves as the inspiration for the "lag" component of the term. The term's first use was found in a Los Angeles Times article on February 13, 1966. Horace Sutton wrote, "If you're going to be a member of the Jet Set and fly off to Katmandu for coffee with King Mahedra, you can count on contracting Jet lag, a debility not unakin to a hangover. Jet Lag derives from the simple fact that jets travel so fast they leave your rhythms behind." The term began gaining popularity soon after, continuing to increase to this day.
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, early awakening, 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 three hour or more hours of time zone change to occur, but some individuals can be affected by a single hour of time zone change which can include 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. During jet lag, there is a shift in the sleep-wake cycle, disrupting the coordinated regulation of the suprachiasmatic nucleus of the hypothalamus. The output of the SCN influences oscillatory sleep and arousal controls, which can later lead to an effect on daily sleep-wake behavior. When travelling across a number of time zones, a person's body clock 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.
Risk factors
Jet lag has a stronger impact when crossing more time zones over a few days. If someone has had jet lag before, then they are likely to have it again. Additional factors include arrival time, age, stress levels, sleep before travel, and use of caffeine or alcohol. Those over the age of 60 are more sensitive to circadian rhythm changes. The chances of jet lag increase with less sleep before a flight and higher stress levels.Additionally, insufficient sleep before a flight can exacerbate jet lag symptoms. A well-rested state prior to travel helps the body adapt more efficiently to new time zones.
High stress levels can also disrupt the body's natural rhythms, making it more difficult to adjust to a new time zone. Stress-induced hormonal changes may interfere with sleep quality and circadian alignment.
Personal factors such as chronotype, genetic predispositions, and overall health can affect how one experiences jet lag. For instance, individuals with a natural tendency to stay up late may find it easier to adjust to westward travel.
Double desynchronisation
Double desynchronisation is the misalignment between: The body's internal clocks and the external environment and the body's central and peripheral circadian clocks.There are two separate processes related to biological timing: circadian oscillators and homeostasis. The master clock of the circadian system is located in the suprachiasmatic nucleus in the hypothalamus of the brain. There are also peripheral oscillators in other tissues and organs, each having their own oscillatory rates that could be synchronized to the SCN oscillatory rate. 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 and entrains its circadian rhythm to the external environment. It is hypothesised that peripheral oscillators respond to internal signals such as hormones, food intake, and "nervous stimuli" and may take longer time to synchronize to local time.
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 master clock SCN with light from the environment earlier. However, their skeletal muscles, liver, lungs, and other organs may adapt at different rates. This internal biological de-synchronisation is exacerbated as the body is not in sync with the environmenta double desynchronisation, which has implications for health and mood.