The rapid development in aircraft technology has made it possible for the air travel industry to offer an inexpensive means of travel for businessmen and vacationers. Compared to other tourism industries, this leads to high growth rates not only with nearby, but also with long-distance destinations. Intercontinental travel involves substantial environmental changes that may have an effect on health and well-being. When degrees of latitude are crossed - as happens with flights in northern or southern direction - the climatic conditions change. During transequatorial flights, even the seasons change. In this way, a traveler who takes off e.g. from Frankfurt with 0°C may encounter an outside temperature of 30°C after 11 hours of flying time when leaving the airplane in Johannesburg. When degrees of longitude are crossed - as with flights in western or eastern direction -, there is a shift in local time. A traveler who takes off from Frankfurt at 10.00 o'clock and comes down in San Francisco after a twelve-hour flight over 9 time zones arrives there at 13.00, although his clock will indicate 22.00.
The phenomenon of jet-lag occurs only in cases in which transmeridian flights (i.e. flights over several time zones) are used in order to reach the destination. Long-distance travels in north-southern direction do usually not lead to jet-lag - even when the journey takes place at night. The symptoms of jet-lag are: sleep disturbances during the night and unusual fatigue, decreased performance and attention by day as well as gastro-intestinal and psychosomatic problems. These symptoms can last for several days or even weeks.
Apart from the passengers of the flight, be they businessmen or tourists, persons who professionally carry out flights over several time zones are affected by symptoms of jet-lag. Aircraft crews (pilots and cabin crew) may suffer particularly from jet-lag, as they are chronically exposed to changes of the local time due to frequent transmeridian flights.
2. Determinanten of the Jet-Lag
In order to understand the reasons for an occurrence of jet-lag, it is necessary to explain which physical and biological mechanisms are important to the process.Basically, three factors are responsible for the occurrence of jet-lag: the natural temporal system, the high velocity of the aircraft and the circadian rhythm (the so-called "interior clock") of humans.
2.1 the Natural Temporal Sytem
The natural temporal system is based on the regular alternation of night and day. The earth revolves on its own axis once every 24 hours. Since the globe is divided into 360 degrees of longitude (meridians), there is a shift of daylight from one degree of longitude to the next of 4 minutes due to this rotation. 15 degrees of longitude are crossed in one hour. Because of this, the earth is divided into 24 time zones, every one of which corresponds to 15 degrees of longitude. In reality, time zones are not strictly divided by meridians, though, but are subject to geographic and national borders. In this way, the Cental European Time (CET), for example, covers countries that would otherwise belong to an earlier (Poland) or later (Spain) time zone. On eastbound flights, the day is shortened and the clock has to be set to a later time for as many hours as time zones are crossed. In reverse, the clock has to be set to an earlier time with flights in a western direction, prolonging the day. Flights bound in an eastern direction are connected with bringing forward the local time ("advance shift"), westbound flights with a delay ("delay shift"). Most international air lanes are located in the Northern Hemisphere, connecting the important industrial nations, i.e. Europe, the USA and Japan, and thus take place on transmeridian routes.
2.2 Aircraft Velocity
The high velocity of aircraft is another reason for jet-lag. If more than 15 to 22,5 meridians per day are not exceeded, the resulting temporal difference of 60 to 90 minutes may possibly be balanced by the adaptability of the human circadian rhythm. For this reason, jet-lag does not occur with travel by ship, train or automobile. With a higher travel velocity, the interior clock cannot balance the difference in time between "old" and "new" local time and is a certain period of time late (with westward flights) or fast (with eastward flights). In fact, examinations of crews of propeller-driven jet aircraf thave already shown that the interior clock is still set to the local time of the take-off location at the time of arrival and the adaption to the new local time begins only with the beginning of sleep. Thus, the use of modern aircraft equipped with significantly higher velocities will not increase the symptoms of jet-lag. However, in times when only few persons undertook transmeridian air passages, they were only known to this small group of flyers. The age of mass tourism changed this situation significantly, exposing millions of people a year to jet-lag. In order to show the problem of high travel velocities and the time of day more clearly, imagine a traveller flying to Alaska for a summer holiday. When he takes off in Hamburg, it is 14.00. When the plane lands in Anchorage, it is still 14.00 due to the time shift, although the plane has been flying for 10 hours. The interior clock is now set to "go to bed", although it is the middle of the day in Anchorage (the destination).
2.3 the Interior Clock
The third determinant is the "biological time" that is determined by the circadian rhythm of humans. Many periodic changes in bodily functions are created by the innterior biological temporal system. Some examples are: periodical fluctuations on a scale of tenths of seconds (brain waves), of one second (heartbeat), of 90 minutes (dream sleep), of 24 hours (sleep-waking-cycle), of 28 days (menstruation cycle) and of one year (spring fatigue). If the period duration is at 24 hours, it is called circadian rhythm. These (daily) temporal fluctuations occur in many bodily functions, e.g. with the body temperature, hormone secretion and the performance. The autonomous determination of these functions have been tested in many studies excluding influences from the outside (e.g. the alteration between light and dark). Here, the duration of the period of the circadian rhythms free from external influence was prolonged to about 24.5 hours. Under normal environmental conditions, the circadian rhythms adjust themselves to the natural daily period of 24 hours. It has been concluded from this that particularly with animals and plants, the alteration of light and dark synchronizes the circadian system to these 24-hours-periodics. In the age of modern technology, this temporal indicator does not have the overwhelming importance for humans that it has for fauna and flora, as e.g. the use of artificial light by-passes the natural light-dark alteration dependent on seasons. Nevertheless, social contacts, cycles of work and rest as well as meal and sleep times are usually distributed quite regularly over the day (excepting shift work) and have an additional function as an indicator of 24-hour time. The biological system regulates the human interior clock and co-ordinates the endogeneous procedures with these external timers. Transmeridian Flights bring a sudden shift of these synchronizing environmental factors with them and thus create a temporal imbalance between the interior clock and the exterior natural temporal system. This imbalance is called the desynchronization of circadian rhythm and natural timers. The circadian rhythm is able to adjust to the new local time, but shows a certain inertia because of which several days or even weeks are needed in order to once again produce a complete synchronization. The inability of the endogenous rhythm to immediately adapt to a new local time after a sudden temporal shift produces the jet-lag.
As a result of this imbalance, a great number of time zone travellers suffer from an impairment of their well-being. These complaints are expressed in automatic functions like e.g. feeling of hunger, sleeplessness, drowsiness, increased fatigue and diminished performance and in gastrointestinal and psychosomatic problems. Due to the inertia of the circadian system in respect to the adaptian to the new local time, the symptoms of jet-lag occurr during unusual and unfavorable times of the day. We will show this using the example of the transmeridian flight from Frankfurt to San Francisco (to and fro). The temporal difference between Frankfurt and San Francisco is nine hours. We presuppose that a complete adjustment to the local time has taken place before the start of the respective flight. The flight is to begin at 10.00 local time in Frankfurt. The landing takes place at 14.00 after 11 hours of flight. Although it is the middle of the day in San Francisco and the people living there are completely active, the traveler will feel tired and will want to go to bed, because his interior clock is set to 23.00 after the arrival at the destination. As the bodily functions are set to "sleep", having dinner would not be a compulsion for the local population. In contrast, the arriving traveler feels hungry during the first nights and is often awake in bed, becuase the interior clock is already set to the following day. By day, he feels tired, drowsy and not really fit for the day. For the flight crew, this situation is particularly unpleasant, since they frequently stay only for a short time at the destinations and possibly have to conduct further flights during times of day unfavorable for the performance.
3. Desynchronization and Resynchronization
The symptoms of jet-lag differ from individual to individual and their strength and duration additionally depend on the direction of the flight (eastward or westward) and on the number of time zones crossed.
3.1 Inter-Individual Differences
Substantial differences in the adjustment of the circadianen rhythm can be observed between individuals. The inter-individual range of covers differences in adjustment from so-called "fast adapters" whose circadian rhythm adapts in a few days to the "slow adapters", whose synchronisation takes two weeks or even more. The following characteristics have been determined as factors influencing the speed of resynchronisation: faster adjustment has been observed with evening types, young persons and with persons whose circadian rhythms are rather unstable, i.e. whose circadian amplitudes are rather low. In contrast, morning types, elderly persons and persons who show stable circadian rhythms with high amplitudes have more problems with the adjustment after time zone flights.
Examining the individual reactions of the processes related to body temperature over several days after a sudden time shift, we can observe that on the days before the time shift, the curves describe a rather orderly state of the circadian temperature rhythm, the daily maximum (acrophase) of which occurs between 15.00 to 17.00 and the minimum of which occurs between 03.00 and 05.00. This regularity breaks down due to the desynchronization caused by the time shift and only reappears after several days (with some probationers needing more time than others). During the examination, the adjustment of the temperature rhythm to the new local time took four to seven days. The duration of the process of adjustment is an indicator for the duration of the jet-lag. The symptomatology at the beginning of the resynchronization is more strongly pronounced and diminishes with increasing duration of the stay.
Time shifts do not only influence the acrophases, but also the daily amplitudes, which are a measure for the periodicity and are calculated from the difference between daily maximum and nocturnal minimum. On the days after the time shift, the amplitudes are decreased by 30 % to 35 % and only later resynchronize themselves to the base values. The reduction of the amplitudes indicate the strength of the jet-lag. The more the amplitude is reduced, the faster the adjustment to the new local time takes place. Howver, a lower amplitude means a stronger jet-lag.
3.2 Asymmetrical Resynchronization
The resynchronization after flights in a western direction happens by way of an extension of the circadian periodics.Should the process of resynchronization be symmetrical, an according shortening of the duration of the period could be expected. This is usually the case with time shifts of up to 6 hours. But an extension was observed with transmeridian flights of longer duration. Instead of a reduction of e.g. 9 hours, an extension of 15 hours may occur ("anti-dromic effect"). Another study showed (with two probationers) that after a time shift of 9 hours in eastern direction, the resynchronization of the temperature rhythm took place by way of a reduction of the duration of the period and was finished after only three days. In contrast, the ryhthm of the the remaining six probationers lasted for an extended period. The consequence of this effect is that the resynchronization takes longer than it does with a reduction (during the study, ten days were needed) and the jet-lag can thus not be overcome quickly. This asymmetry in the resynchronization is caused by the behavior of the internal clock, which has a periodic duration of more than 24 hours and can thus adapt better to an extension of the day than to a reduction.
3.3 Internal Dissociation
The process of resynchronization has been demonstrated by the body temperature. Other bodily functions may react differently to sudden alterations of time. The adaption of the circadian rhythm of different hormones or electrolytes may progress slower or faster. While the rhythm of secretions of sodium and of adrenalin adjusts rather quickly (3 to 6 days), the adjustment of secretions of cortisol and of potassium takes significantly longer, i.e. 8 days or even longer. These different reactions to an alteration of the timers is called internal dissociation. As the normal temporal relation between the different rhythms is disturbed by this process, it contributes to jet-lag.
Additionally, a different adjustment for different rhythms may take place with critical time shifts (7 to 11 hours in eastern direction) ("resynchronisation by partition"). This means that in one person, one part of the bodily functions resynchronizes by way of a reduction and another part by way of an extension.
Apart from the two phenomenons described above and from inder-individual differences, we can formulate a general guide for the average speed of adjustment and thus for the duration of the jet-lag (presupposing that no specific measures shortening the duration are taken). For this, we have selected the body temperature as a basis for the process of resynchronization. According to this, the adaption after a transmeridian flight progresses exponentially and the remaining desynchronization is halved every two days.
3.4 Sleep and Disturbances of Sleep
The physiological symptoms after transmeridian flights best known are not the effects of desynchronization of the circadian rhythm, but the disturbances of sleep which most people believe they suffer from without recognizing the processes of circadian rhythms as the cause for this. The usual sleep complaints confirmed by examinations of probationers and pilots are difficulties in falling asleep, repeated spontaneous waking during the night and sleep deficit caused by early waking in the morning.
In order to record these disturbances of sleep objectively, many examinations in flight medicine wer conducted on passengers and pilots during their round trips on transmeridian routes using polysomnography.
The technology for recording and assessing sleep by measurements of the electroencephalogram (EEG) has been standarized for 30 years. According to this, the sleep-EEG can basically be divided into four stages of orthodox sleep (stage 1 to 4) and one stage of paradoxical sleep or REM-sleep ("Rapid Eye Movement"). Stages 1 and 2 are chracterized mainly by fast waves of brain electricity (12-14 Hz) and occur particularly towards the end of the sleep. They represent the stages of a light sleep. In stages 3 and 4, low frequencies (2-4 Hz) mainly occur. These can be observed during deep sleep. In the waking condition, all frequencies occur (1-30 Hz). With eyes closed, frequencies between 8 and 12 Hz are dominant. During normal sleep, the transitions between the individual sleep stages are rather regular and have a pattern that is repeated approx. every 90 minutes. After a relatively short period of approx. 10 minutes in stages 1 and 2, the stages of deep sleep follow. The stages of deep sleep last approx. 5 to 10 minutes during stage 3 and 10 to 60 minutes during stage 4. After this, an REM period of up to 40 minutes follows. In the beginning of normal sleep (during a (ultradian) cycle of 90 minutes), the portion of deep sleep is rather high while the portion of REM sleep is relatively low. In the further process, deep sleep decreases and REM sleep increases in a cycle of 90 minutes. With young adults, the portion of deep sleep of the total sleep is approx. 20 %. With increasing age, the portion of deep sleep decreases. Because of this, elderly persons hardly have any stage 4 sleep at all. Simultaneously, the number and duration of waking periods during sleep increases.
The sequence of the sleep stages changes after transmeridian flights. The differences are more strongly pronounced after flights in an eastern direction than they are after flights in a western direction. After flights in western direction, problems with sleeping through the night appear which manifest themselves in an increased number of wakings during the second half of the night. Due to the delay in sleep of several hours (assuming the traveler does not go to bed directly but waits for the night) caused by the time shift, a certain sleep deprivation occurs, faciliating falling asleep. In this way, stages 1 and 2 are processed quickly and deep sleeps sets in faster and may take longer. The structure of sleep is changed by the earlier occurence of long periods of REM sleep, as well. During the second half of sleep, increased portions of periods of light sleep and an increase in the frequency of periods of waking occur. This is caused by the circadian rhythm that has not yet adapted to the new local time and thus activates the bodily functions for the alleged day. In the course of several days, sleep and the ultradian structure will normalize. Here, the duration of the adjustment will once again depend on the temporal difference. Consequently, the duration after a flight in western direction crossing 6 time zones will be 2 to 4 days and 4 to 6 days with 12 time zones.
In contrast, sleep is frequently altered strongly after a flight in eastern direction. While only minor sleeping plroblems due to sleep deprivationl occur during the first night (the reason for this is the flight in eastern direction, which usually takes place at night and during which sleep seldomly occurs), it is significantly disturbed during the following nights. The reasons for this are difficulties in falling asleep, fragmentation of sleep and the unusually long periods of dreaming during the first half of the night. Again, this is caused by the interior clock 's lack of adjustment . The interior clock is still set to waking and to being active, although night is falling at the location of destination.
4. Fatigue and Performance
Disturbances of the circadian system and of sleep have effects on the fatigue and performance by day. For crew members on duty during long-distance flights, this means that they are not always optimally rested at the beginning of their work and that periods of increased fatigue and decreased attention may occur. Examinations on several transmeridian routes have shown that sleep deprival and disturbances of rhythm may lead to a critical assessment of the state of fatigue and to the occurrence of micro-sleep. The state of fatigue was registered by use of questionnaires and the occurence of micro-sleep was recorded by use of a continual measurement of the EEG in waking condition during the flights. During round trips between (DUS) and Atlanta (ATL), twelve pilots were examined in the two-man cockpit. While no critical results were observed on the outward flight DUS-ATL (flight by day) that took approx. 11 hours of flight duty, some of the pilots were so tired during the return flight ATL-DUS that their fatigue was assessed as critical for the performance. This was caused by the circadian rhythm (the temporal difference between DUS and ATL was 6 hours) and by the time of the take-off in Atlanta (midnight), which did not leave room for sufficient sleep before the return flight.
From the results of these and further examination on pilots (on different transmeridian and north-south routes), we can conclude that substantial strain during long-distance flights is caused by the irregularity of the working hours. The main factors of strain here are (1) the night-work, because the performance by night is diminished by the progress of the circadian rhythm and by sleep deprival, (2) the jet-lag that doesn't allow for the crew to rest sufficiently before going back on duty, (3) a long period of flight duty and the impossibility of taking recreational breaks in case of the two-man crew.
In conclusion, these alterations can be determined as typical consequences of the desynchronization after flights over time zones:
- A temporal shift of the rhythm curves and sleep profiles (during early times of day after flights in western direction, during later times of day after flights in eastern direction);
- A differently fast adjustment of the different bodily functions;
- Changes in the circadian amplitudes;
- Faster adjustment to the local time after flights in western direction than after flights in eastern direction.