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 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 (most long distance travel was done by sea and rail).
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. Jet lag started to be associated 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 (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 three 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. Frequent traveller to Australia, British businessman Alistair McAlpine (1942–2014), wrote of the benefits of being awake at unusual hours in a 2002 memoir:
Possibly the only advantage of jet lag is that you are fully awake at times of the night when you would otherwise be unaware there was anything worth waking to see: the last hours of full moon, for instance, as it hovers over Sydney Harbour (..) Then there is the freshness as you walk in the Botanical Gardens, a freshness that only comes with dawn, a few moments caught between the stifling heat of antipodean night and the blazing sun of day.
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