Jet lag

A Jet lag is a physical reaction to a disturbance of the sleep-wake rhythm that occurs after transmeridian flights. The circadian rhythm of the body cannot adapt quickly enough to the time change, which can result in a number of psychological and physical complaints.

What is the jet lag?

A disruption of the sleep-wake rhythm that occurs after long-haul flights that span multiple time zones is known as jetlag. The word is made up of the English word “jet” (jet aircraft) and “lag” (time difference).

In the current edition of the International Statistical Classification of Diseases and Related Health Problems (ICD-10), the disorder is classified under number F51.2 as "Inorganic disorder of the sleep-wake rhythm". According to the classification system for sleep disorders (ICSD-2), the phenomenon is classified as a “circadian sleep-wake rhythm disorder, jet lag type”.

Traveling across several time zones causes the biorhythm to lose its rhythm and is no longer in sync with the current local time. Natural rhythms of the body get mixed up due to unusual changes between light and dark and changed eating and sleeping times.

Since the internal clock sometimes does not adjust to the new local time quickly enough, physical and psychological complaints can occur that can last for two to fourteen days. The symptoms of jet lag are sleep disorders, excessive tiredness, reduced performance during the day and psychosomatic problems.

Function & task

The biological rhythm of mammals (including humans) is determined by a series of external timers that regulate the cycle of many body functions (such as body temperature, hormone release and blood pressure).

The internal clock that determines the circadian rhythm is located in the suprachiasmatic nucleus, part of the hypothalamus. The most important exogenous timers of the internal clock include the alternation of day and night, the time of meals, bedtime and social contacts. If these timers run normally and regularly, the internal clock usually runs every 24 hours, synchronized with external conditions, and the biological system coordinates endogenous processes of the body with exogenous processes.

On transmeridian flights the external environmental factors suddenly shift and the internal clock is not able to compensate for the temporal imbalance between the circadian rhythm and the external time system quickly enough. A time difference of 60 to 90 minutes can be compensated relatively easily by the circadian rhythm. However, if the travel speed and thus the time difference increase, the internal clock can sometimes no longer adapt and either lags behind or moves ahead.

The extent of jet lag is determined by the direction of flight and is less when traveling to the west than when traveling to the east. This is because it is usually easier for people to stay awake longer than it is to fall asleep earlier and wake up earlier. Flights to the west require extended clock phases, i. This means that the day is "postponed" and sunrise and sunset are delayed. For air travelers, this means that they have to stay up longer at their destination.

For flights to the east, on the other hand, the clock phases are shortened and the day is "moved forward", sunrise and sunset occur earlier. Air travelers therefore have to go to bed earlier and get up earlier. Those who fly from Frankfurt to New York, i.e. in a westerly direction, have a flight time of around six hours. If the arrival time in New York is 6 p.m., it is already midnight in Germany due to the time difference. In order to adapt to the local time in New York, you only have to stay awake a few hours longer and the changeover is relatively easy. On the return flight, however, the clock in Frankfurt must be put forward. If the arrival time is 11 p.m. local time, the internal clock is still set to 5 p.m., although it is already bedtime in Frankfurt.

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Illnesses & ailments

The lack of synchronization between the internal clock and external circumstances can manifest itself in many symptoms. The duration of the change and the associated symptoms depend on the extent of the time difference, the age of the person concerned and the state of health.

Evening types, younger people and people whose circadian rhythms are more flexible, generally report fewer symptoms and show a faster adaptation of the circadian rhythm.

Morning types, elderly people and people who have a strong routine and a very regulated daily routine are more strongly influenced by the time difference and thus also experience jet lag more strongly. It can take two to fourteen days for the circadian rhythm to adjust again. In general, an adjustment period of around half a day per time zone flown over is assumed.

As a result of the imbalance between the circadian rhythm and local time, a number of complaints can arise. Travelers report impaired well-being, excessive fatigue, reduced performance during the day, dizziness, mood swings, feelings of hunger or loss of appetite at inconvenient times and a number of other psychosomatic and gastrointestinal problems.

The most common complaints of jet lag, however, are sleep disorders such as difficulty falling asleep and staying asleep, early waking up in the morning and insomnia. The sleep rhythm is disturbed and the sleep stages change after long-haul flights. After flights to the west, slight problems staying asleep occur more frequently due to the extended cycle phases, while flights to the east manifest themselves in particular in difficulty falling asleep due to the shortened cycle phases.

The disturbances of the circadian rhythm and the sleep disturbances in turn affect daytime sleepiness and cognitive performance.

This has significant consequences not only for vacationers, but also for flight crew members and shift workers. They often have to go to work unrested and then have to struggle with increased fatigue and reduced performance. The health consequences for people who are professionally exposed to a constant change of sleep-wake rhythm can result in chronic illnesses.