Jet lag is a physical reaction to a disturbance in the sleep-wake rhythm that occurs after transmeridian flights. The body’s circadian rhythms cannot adjust quickly enough to the time change, which can result in a number of psychological and physical ailments.
What is jet lag?
Jet lag is a physical reaction to a disturbance in the sleep-wake rhythm that occurs after transmeridian flights. A disturbance in the sleep-wake rhythm that occurs after long-haul flights that span multiple time zones is called jet lag. The word is composed of the English word “jet” (jet airplane) 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 the number F51.2 as “Non-organic disorder of sleep-wake rhythm.” According to the Classification System for Sleep Disorders (ICSD-2), the phenomenon is classified as “Circadian sleep-wake rhythm disorder, jet lag type.” Traveling across multiple time zones throws the biorhythm out of sync with the current local time. Natural rhythms of the body get confused by unaccustomed changes between light and darkness and altered eating and sleeping times. Because the internal clock sometimes does not adjust quickly enough to the new local time, physical and psychological complaints can occur that can last from two to fourteen days. Symptoms of jet lag have been described as sleep disturbances, excessive fatigue, reduced daytime performance and psychosomatic problems.
Function and task
The biological rhythm of mammals (including humans) is determined by a series of external timers that regulate the cycle of many bodily functions (such as body temperature, hormone secretion, and blood pressure). The internal clock that determines circadian rhythm is located in the nucleus suprachiasmaticus, a part of the hypothalamus. Key exogenous zeitgebers of the internal clock include the alternation of day and night, the timing of meals, bedtimes, and social contacts. If these timers run normally and regularly, the internal clock usually runs in synchrony with external conditions on a 24-hour basis, and the biological system aligns endogenous processes of the body with exogenous processes. On transmeridian flights, external environmental factors shift suddenly and the internal clock is unable to balance the temporal imbalance between circadian rhythms 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 becomes greater, the internal clock is sometimes unable to adjust and either falls behind or goes ahead. The severity of jet lag is determined by the direction of flight and is less pronounced when traveling west than when traveling east. This is because it is generally easier for people to stay awake longer than to fall asleep earlier and get up earlier. Westbound flights require extended clock phases, meaning that the day is “pushed back” and sunrise and sunset are delayed. For the air traveler, this means staying up longer at the destination. For flights to the east, on the other hand, the cycle phases are shortened and the day is “shifted forward,” with sunrise and sunset occurring earlier. Air travelers therefore have to go to bed earlier and get up earlier. Anyone flying from Frankfurt to New York, i.e. in a westerly direction, has a flight time of about six hours. If the arrival time in New York is around 6 p.m., it is already midnight in Germany due to the time difference. In order to adjust 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, on the other hand, the clock in Frankfurt has to be set ahead. If the arrival time is around 11 p.m. local time, the internal clock is still set to 5 p.m., even though it is already bedtime in Frankfurt.
Diseases and ailments
The lack of synchronization between the inner clock and external circumstances can manifest itself in many symptoms. The duration of the changeover and the symptoms associated with it depend on the extent of the time difference, the age of the person affected and his or her state of health.Evening types, younger people, and people whose circadian rhythms are more flexible generally report fewer symptoms and exhibit faster circadian rhythm adjustment. Morning types, older people, and people who have strong routines and very regular daily routines are more affected by the time difference and thus experience more jet lag. It can take two to fourteen days for the circadian rhythm to readjust. Generally, an adjustment period of about half a day per time zone flown over is assumed. As a result of the imbalance between circadian rhythm and local time, a number of complaints can develop. Travelers report impaired well-being, excessive fatigue, decreased daytime performance, dizziness, mood swings, feelings of hunger or loss of appetite at inconvenient times, and a number of other psychosomatic and gastrointestinal problems. However, the most common complaints of jet lag are sleep disturbances such as difficulty falling asleep and staying asleep, early morning awakenings, and insomnia. Sleep rhythms are disturbed and sleep stages change after long-haul flights. After flights to the west, slight sleep-through problems occur more frequently due to the extended clock phases, while flights to the east manifest themselves particularly in sleep-onset disorders due to the shortened clock phases. The disruption of circadian rhythms and sleep disturbances in turn affect daytime sleepiness and cognitive performance. This has significant consequences not only for vacation travelers, but also for flight crew members and shift workers. They often have to report for duty unrested and subsequently struggle with increased fatigue and reduced performance. The health consequences for people whose jobs expose them to constant changes in sleep-wake rhythms can result in chronic illnesses.
