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Reilly, T. (1998). Travel: Physiology, jet-lag, strategies. In: Encyclopedia of Sports Medicine and Science, T.D.Fahey (Editor). Internet Society for Sport Science: http://sportsci.org. 12 July 1998.
Symptoms
Circadian
Desynchronization
Reducing Jet
Lag
References
The feelings of disorientation encountered as a result of crossing time zones are known as jet lag. Symptoms include fatigue and general tiredness, inability to sleep at night, loss of concentration, loss of drive, headaches and general malaise. Jet-lag occurs when biological rhythms are disrupted as a result of rapid transitions across multiple time-zones. Such desynchronization of rhythms also occurs in nocturnal shift work employees who transfer to night shifts.
A classical rhythm is represented by a sine wave, fluctuations occurring cyclically about a mean value rising to a peak and half a cycle later dropping to a trough. Twenty-four hour rhythms are known as circadian (about a day). An example of a biological rhythm is core temperature. We can find the characteristics of rhythms using the mathematical technique of cosinor analysis to determine the mean, the peak, the amplitude, and the time the peak occurs (the acrophase).
Following a journey across multiple time zones the body's rhythms at first retain the characteristics of their point of departure. However, the new environment forces new influences on these cycles, the main factors being the time of sunrise and onset of darkness. The body attempts to adjust to this new context but core temperature is relatively sluggish in doing so. As a rough guide it takes about one day for each time zone crossed for body temperature to adapt completely. The individual may have difficulty in sleeping for a few days, but activity and social contact during the day help in accelerating the adaptations of the arousal rhythm. Thus arousal adjusts more quickly than does body temperature to the new time zone. Until the whole spectrum of biological rhythms adjusts to the new local time, the performance of exercise may be below par.
Allowing for individual differences, the severity of jet lag is affected by a variety of factors. In general, the greater the number of time zones crossed, the more difficult it is to cope. A 2-hour phase shift may have marginal significance but a 3-hour shift (e.g. British or Irish teams traveling to play European football matches in Russia or Turkey or a Californian sports team traveling to play on the east coast of the US) will entail desynchronization to a substantial degree. In such cases the flight times time of departure and time of arrival - may determine the severity of symptoms.
The severity of symptoms may be worse 2-3 days after arrival than on the day immediately following disembarkation. Symptoms then gradually abate, but can still be acute at particular times of day. There will be a window of time during the day when the period of high arousal associated with the time zone just left overlaps with the arousal high point at the new local time. This window may be predicted in advance and should be utilized for timing of training practices in the first few days at destination. Our observations on footballers traveling from Britain to the South Pacific indicate that morning training sessions suit players best over the first few days. Such a practice has also proved successful for cross-country runners.
The direction of travel affects the severity of jet lag. It is easier to cope with flying in a westward direction compared to flying eastward. In flying westward the normal cycle is temporarily lengthened and body rhythms can extend in line with their natural freewheeling period of about 27 hours and thus catch up. Observations on traveling to Korea (9 hours in advance of British Summer time) and Malaysia (7 hours in advance of British Summer time) are that periods of 9 and 6 days respectively may be inadequate for jet lag symptoms to disappear. In contrast re-adaptation is more rapid on returning to Britain. When time zone shifts approach the maximum 12-hour change, there may be little difference between eastward and westward travel.
The direction of travel can be a relevant consideration when going from coast to coast to compete in North America. When going eastward the mean performance is depressed more and the peak performance declines more dramatically than is the case on traveling westward. The reason for westward travel being easier is because the natural period of circadian rhythms is greater than 24 hours, so each rhythm adapts more quickly when the day is artificially lengthened. Altering training times for a few days prior to travel to take into consideration the time of competition in another time zone is known to be beneficial.
Young individuals have a better tolerance to desynchronization of rhythms, owing to a better regulation of biological clocks. Physical fitness also seems to play a role: active subjects demonstrate higher amplitudes in existing rhythms than age-matched controls, a difference indicative of superior regulation. But other than these factors, there has been little success in attempting to predict good and poor adapters to long haul flights. Furthermore, the fact that an individual escapes lightly from symptoms on one occasion is no guarantee that he or she will do so again on the next visit.
Scheduling Travel
If it is possible to do so, flights should be scheduled so that athletes arrive well in advance of competition. One day for each time zone crossed does leave a cushion of safety, even traveling eastward. The time for adaptation may be shortened by exploiting the external factors that reset biological clocks: rest/exercise, darkness/ light, meals and social influences. The key is to tune in straight away to the external influences of the new environment.
It may be beneficial to shop around to find the most convenient travel schedules. Consider departure from regional airports if appropriate and also alternative carriers. The routines prior to departure, on the plane, and after arrival, can be planned once the itinerary is established. In consequence coping with jet lag will not be the hit or miss affair it might otherwise be.
Before and During Travel
On lengthy journeys it is unlikely that any maneuvers will eliminate jet lag, but with careful planning the symptoms can be attenuated. In the week prior to departure it may be possible to adjust the time of arising and going to bed, the adjustment depending on the direction of flight. An alteration of more than two hours is likely to be unproductive, since this would interfere with the pattern of social and domestic engagements during the day. Besides, the major synchronizer of human circadian rhythms--natural daylight--remains unaltered.
The evidence from simply altering time of retiring to sleep is that shifting the sleep-wake cycle does alter rhythms in accord with the direction of the shift, but motor performance is compromised during the course of such adaptive changes. Manipulating the sleep-wakefulness rhythm prior to traveling abroad to compete therefore may not be the best strategy.
Once flight times are known, a routine on the plane may be planned. In day time flights it will be necessary to stay awake, keep mentally active and perhaps watch the in-flight movie. On long haul flights that entail traveling during the night it will be necessary to get some sleep on the plane. The timing of this should be decided in advance so that some meals on board can be missed. Transit or transfer episodes en route should be taken into consideration. It is a good strategy to set one's watch to local time at the next point of landing, once on board the plane: in a single haul flight this would be the local time of the country of destination. The important thing is that the traveler mentally tunes in to the new local time straight away and adjusts behavior accordingly.
To compensate for the dry air on board flight, copious rehydration is advised. Fruit juices are best, fizzy drinks should be avoided. Alcohol should not be taken, since it acts as a diuretic (increases urine production) and also affects the normal circadian rhythm in renal function. Caffeine in coffee also stimulates water loss, and its arousal effect on the central nervous system means it should not be taken if sleep is desired. One suggestion is that the last meal prior to the time allotted for sleep should be high in carbohydrates and low in protein in order to induce drowsiness. Carbohydrates provide the substrate for serotonin, a neurotransmitter that regulates sleep. Caffeine and a low-carbohydrate high-protein breakfast would help raise the level of arousal and prevent a relapse into sleep.
Athletes may feel stiff or cramped because of their restrained posture on board flight. They can perform isometric exercises for arms, trunk or legs while in their seats. It is even better to walk down the aisle of the plane and occasionally do flexibility or stretching exercises at the back of the plane.
British sports teams traveling to Australia have used sleeping pills to induce sleep while on board. Although such drugs as benzodiazepines are effective in getting people to sleep, they do not guarantee a prolonged period asleep. Besides, they have not been satisfactorily tested for subsequent residual effects on motor performances such as sports skills. They may also be counter-productive if administered at the incorrect time. A prolonged nap at the time the individual feels drowsy (presumably at the time that he or she would have been asleep in the time zone departed from) simply anchors the rhythms at their former phases and so resists the adaptations to the new time zone.
On reaching the country of destination a key factor is to fit in immediately with the phase characteristics of the new environment. Athletes should already have worked out the local time for their disembarkation. There may be other environmental factors to consider such as heat, humidity or even altitude.
Having traveled westward, players may be allowed to retire to bed early. Early onset of sleep will be less likely after an eastward flight. In this case a light training session on that evening would be helpful in instilling local cues into the rhythms. Besides, there is some evidence that exercise does speed up the adaptation to a new time zone.
For the first few days in the new time zone, training sessions should not be all-out efforts. Skills requiring fine co-ordination are likely to be impaired and this might lead to accidents or injuries if, for example, games players conducted sessions with the ball too strenuously. Where a series of tournament engagements is scheduled, it is useful to have at least one friendly match during the initial period (before the end of the first week) in the overseas country.
In this period of adaptation a few caveats are noted. Alcohol taken late in the evening is likely to disrupt sleep and so is not advised. The alternation of feasting and fasting recommended for commercial travelers in the USA is unlikely to gain acceptance among footballers. Nevertheless they could benefit from biasing the macronutrients in their evening meal largely towards carbohydrates. These would include vegetables with a choice of chipped, roast or baked potatoes, pasta dishes, rice and bread. These should include sufficient fiber to safeguard against constipation.
In the early days in the new country athletes should be discouraged from taking prolonged naps. A nap at the time they would have been asleep had they stayed at home would make subsequent sleep more difficult and retard the adjustment of the major biological clocks to the new regimens. Exposure to bright light, preferably natural daylight, is a useful antidote to drowsiness in such circumstances.
Taking drugs can alter biological clocks, depending on the time they are taken. Caffeine (in coffee) and theophylline (in tea) are stimulantsm of the central nervous system. Taken in the evening they would help in recovery after flying eastward and in the afternoon after flying westward.
The minor tranquilizers, benzodiaepines, affect neurotransmitters that have a role in arousal. These include serotonin, noradrenaline, acetylcholine and gamma-aminobutyric acid (GABA). As already indicated, they are effective in inducing sleep but not necessarily good in ensuring the state of sleep. Also hangover effects cannot be excluded, even in some so-called short-acting hypnotics.
Administration of melatonin, which is a pineal gland hormone, has shown good results in treating Scandinavian patients suffering depression in the winter. This clinical condition is known as seasonal affective disorder. The lack of stimulus from natural light for the pineal gland to secrete melatonin is a recognized cause of this entity. Prolonged exposure to bright artificial light has also proved effective in those patients. Subjects who took melatonin tablets on a trip from Britain to Australia experienced a reduction in jet-lag symptoms, improvements in sleep quality, and a faster readjustment of cortisol and melatonin rhythms.
These findings received further support from results of a simulated eastward flight across nine time zones. It seems that administration of melatonin in the afternoon or evening causes phase advances of circadian rhythms whereas a phase delay (required after a westward flight) results from administration at night or early morning. However, toxic side-effects of this substance cannot be discounted and besides, it is not yet readily available in tablet form.
There is also a suggestion that the amino acid tryptophan is helpful in coping with jet-lag. It is a precursor of the sleep hormones, but there is no evidence that it improves the quality of sleep. Besides, it has achieved bad publicity in the early 1990s due to impurities being found in commercially available products and its use is no longer recommended.
It is more effective to use light or behavioral measures to resynchronize circadian rhythms. Natural daylight and bright artificial light help to increase or maintain arousal as well as contribute to returning endogenous rhythms. For athletes exercise is a powerful resynchronizor. It stimulates catecholamines and alertness. It is recommended, even on the day of arrival, except late in the evening local time. Exercise at a light intensity is adequate for stimulating resynchronization of rhythmic characteristics as exercise that is too strenuous may disrupt rather than promote sleep.
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