This is an excerpt from Ergonomics in Sport and Physical Activity by Thomas Reilly.
Elite athletes are regularly called upon to travel large distances to participate in international or interclub competitions. Teams may also participate in closed-season tournaments or friendly games overseas as part of preseason training. Such engagements are made possible by the speed of contemporary air flight. Although international travel is routine nowadays for recreational purposes, it is not without attendant problems for the traveling athlete, which should be recognized in advance.
Many athletes have their regular routines disrupted when they travel abroad. They may be particularly excited about the trip or worried about planning for the departure. Depending on the country to be visited, visas and vaccinations may be required. Professional teams usually have arrangements made for them by their administrative and medical staff. These arrangements extend to coping with formal procedures at departure and disembarkation and avoiding any mix-ups in dealing with ground staff and security controls.
Having arrived safely at the destination, the athlete may suffer travel fatigue, loss of sleep (depending on flight times), and symptoms that have come to be known as jet lag. This term refers to the feelings of disorientation, light-headedness, impatience, lack of energy, and general discomfort that follow traveling across time zones (see highlight box). These feelings are not experienced with traveling directly northward or southward within the same time zone when the passenger simply becomes tired from the journey or stiff after a long stay in a cramped posture. Jet lag may persist for several days after arrival and can be accompanied by loss of appetite, difficulty in sleeping, constipation, and grogginess. Although individuals differ in severity of symptoms they experience, many people simply fail to recognize how they are affected, especially in tasks requiring concentration, situation awareness, and complex coordination.
The body’s circadian rhythm at first retains the characteristics of the point of departure following a journey across multiple time zones. The new environment soon forces new influences on these cycles, mainly the time of sunrise and onset of darkness. Endogenous circadian rhythms such as core temperature and other measures are relatively slow to adjust to this new context. It takes about one day for each time zone crossed for core temperature to adapt completely. Sleep is likely to be difficult for a few days, but exogenous rhythms such as activity, eating, and social contact during the day help to adjust the sleep-wake rhythm. Arousal state adapts more quickly than does body temperature to the new time zone. Until the whole range of biological rhythms adjust to the new local time and become resynchronized, athletes’ performance may be below par.
The severity of jet lag is affected by a number of factors besides individual differences. The greater the number of time zones traveled, the more difficult it is to cope with changes. A 2 hr phase shift may have marginal significance, but a 3 hr shift (e.g., British or Irish teams traveling to play opponents in Russia, or American athletes traveling coast to coast within the United States) will cause desynchronization to a substantial degree. In such cases the flight times-time of departure and time of arrival-may determine the severity of the symptoms of jet lag. Training times might be altered to take the direction of travel into account. Such an approach was shown to be successful in American football teams traveling across time zones within the United States and scheduled to play at different times of day (Jehue et al., 1993).
When journeys entail a 2 to 3 hr time-zone transition and a short stay (2 days), it may be feasible to stay on "home time." Such an approach is useful if the stay in the new time zone is 3 days or less and adjustment of circadian rhythms is not essential. This approach requires that the time of competition coincide with daytime on home time. If this is not the case, then adjustment of the body clock is required. A European team that is to compete in the morning in Japan or in the evening in the United States will require an adjustment of the body clock, because these timings would otherwise be too difficult to cope with.
Symptoms of jet lag recede after the first 2 or 3 days following arrival but may still be acute at particular times of day. There will be a window during the day when time of high arousal associated with the time zone departed from and the new local time overlap. This window may be predicted in advance and should be used for timing of training practices in the first few days at the destination.
The direction of travel influences the severity of jet lag. Flying westward is easier to tolerate than is flying eastward. On flying westward, the first day is lengthened and the body’s rhythms can extend in line with their natural free-wheeling period of about 25 hr and thus catch up. Traveling to Japan (9 hr in advance of British Summer Time) and Malaysia (7 hr in advance of British Summer Time) requires more than 9 and 7 days, respectively, for jet lag symptoms to disappear in some individuals. In contrast, readjustment is more rapid on returning to Britain from the east (Reilly, 2003). However, when time zone shifts approach near-maximal values (e.g., a 10-12 hr change) there may be little difference between eastward and westward travel and the body clock is likely to adjust as if the latter had occurred (Reilly et al., 2005).
Sleeping pills have been used by some traveling athletes to induce sleep while on board flight. Drugs such as benzodiazepines are effective in getting people to sleep but they do not guarantee a prolonged period asleep. They were ineffective in accelerating adjustment of the body clock in a group of British Olympic athletes traveling to the United States (Reilly et al., 2001). Besides, these drugs have not all been satisfactorily tested for subsequent residual effects on motor performances such as sport skills. They may in fact be counterproductive if administered at the incorrect time. Nonbenzodiazepine sedatives such as zopiclone and zolpidem have fewer side effects and minimal interference with normal sleep architecture (Lemmer, 2007). Melatonin is one substance that can act directly on the body clock as well as being a hypnotic, but the timing of administration is critical. Travelers between the United Kingdom and Australia, a journey that can elicit the most severe jet lag symptoms, were found to have no benefit from melatonin (Edwards et al., 2000). Melatonin administered in the few hours before the trough of body temperature will have a phase-advance effect whereas if administered in the hours after this trough will delay the circadian rhythm. Ingestion of melatonin at other times will have no chronobiotic effect but will help to induce drowsiness. Drugs do not provide an easy solution to preventing jet lag, and a behavioral approach can be more effective in alleviating symptoms and hastening adjustment (Reilly et al., 2005).
The timing of exposure to bright light is key in implementing a behavioral approach. Light demonstrates a phase-response curve, opposing the effects of melatonin (Waterhouse et al., 1998). Exposure to natural or artificial light before the trough in core temperature promotes a phase delay, whereas a phase advance is encouraged by light administered after this time, meaning "body clock time." Exposure to light at 10 p.m. in Los Angeles following a flight from London would promote a phase advance on the first night rather than the required phase delay, administration occurring after the trough in core temperature (Waterhouse et al., 2007). Where natural daylight cannot be exploited, artificial light from visors or light boxes can be effective for phase-shifting purposes; these commercially available devices have been used in treating seasonal affective disorder found among natives of northern latitudes during the winter seasons when the hours of daylight are limited. The malaise is not a common affliction among athletes.
The athlete should adjust as soon as possible to the local daytime and nighttime in the new environment. Focusing on the local time for disembarkation can help in planning the rest of the daily activity. Natural daylight inhibits melatonin and is the key signal that helps to readjust the body clock to the new environment. There may be other environmental factors to consider such as heat, humidity, or even altitude.
A phase delay of the circadian rhythm is required after traveling westward, and visitors may be allowed to retire to bed early in the evening. Early onset of sleep will be less likely after an eastward flight. In this case, a light training session on that evening will instill local clues into the rhythms. Exercise can hasten the adaptation to a new time zone, and a light training session on the afternoon of arriving in the United Kingdom after a flight has proved beneficial (Reilly, 1993). Training in the morning is not recommended after a long-haul, eastward flight because it exposes the individual to natural daylight and could delay the body clock rather than promote the phase adjustment required in this circumstance. This strategy of avoiding morning sessions until it was deemed appropriate was used by British Olympic athletes arriving in Australia for the Sydney Olympics in 2000.
Exercise should be light or moderate in intensity for the first few days in the new time zone, because training hard while muscle strength and other measures are impaired will not be effective (see figure 4.3). Skills requiring fine coordination are also likely to be impaired during the first few days, and this might lead to accidents or injuries if technical training sessions are conducted too strenuously. When a series of tournament engagements are scheduled, it is useful to have at least one friendly competition before the end of the first week in the overseas country. Naps should be avoided for the first few days because a long nap at the time the individual feels drowsy (presumably at the time he or she would have been asleep in the time zone just departed from) anchors the rhythms at their former phases and so delays the adaptations to the new time zone.
Some precautions are necessary during adjustment to the new time zone. Alcohol taken late in the evening is likely to disrupt sleep and so is not advised. Normal hydration levels may be reduced following the flight because of respiratory water loss in the dry cabin air, and so fluid intake should be increased. A diet recommended for commercial travelers in the United States entailed use of protein early in the day to promote alertness and carbohydrate in the evening to induce drowsiness. This practice is unlikely to gain acceptance among athletes, although they could benefit from avoiding large evening meals. The evening meal might include vegetables with a choice of chipped, roasted, or baked potatoes; pasta dishes; rice; and bread with sufficient fiber to reduce the risk of becoming constipated.
By preparing for time zone transitions and the disturbances they impose on the body’s rhythms, the athlete can reduce the severity of jet lag symptoms. There has been little success in attempting to predict good and poor adaptors to long-haul flights. The fact that a person feels relatively unaffected on one occasion is no guarantee that she will do so again on the next visit. Regular travelers benefit from their experiences and develop personal strategies for coping with jet lag (Waterhouse et al., 2002). The disturbances in mental performance and cognitive functions have consequences not only for athletes but also for training and medical staff traveling with them, who are also likely to suffer from jet lag symptoms. The long periods of inactivity during the plane journey may lead to the pooling of blood in the legs and in susceptible people cause a deep-vein thrombosis. Moving around the plane periodically during the journey, say, every 2 hr, and doing light stretching exercises are recommended. Travelers should also drink about 15 to 20 ml extra fluid per hour, preferably fruit juice or water, to compensate for the loss of water from the upper respiratory tract attributable to inhaling dry cabin air (Reilly et al., 2007b). Without this extra fluid intake, the residual dehydration could persist into the early days in the new time zone.
Read more about Ergonomics in Sport and Physical Activity.