índices Financieros

Bright light (that is, of an intensity found naturally but not normally indoors) can adjust the body-clock. The timing of exposure is crucially important (Minors et al., 1991) and is the opposite of that for melatonin ingestion; thus, bright light in the morning (05:00–11:00 hours) on body time advances the clock and bright light in the evening (21:00–03:00 hours) on body time delays it. As part of this treatment, there are also times when light should be avoided (those times which produce a shift of the body-clock in a direction opposite to that desired). Table 3 gives times when light should be sought or avoided after different time-zone transitions; the timing will vary as the body-clock adjusts.

104 Reilly, Atkinson and Waterhouse

Even though ‘bright light’ is of an intensity normally not achieved in domestic or interior lighting, light boxes and visors are now available commercially that produce a light source of sufficient intensity. Light visors, in particular, might prove useful.

Since outdoor lighting is the obvious choice, it would be natural, therefore, to consider training outdoors—such as jogging, a brisk walk, a swim or a light training session—when light is required, and to relax indoors when it should be avoided. This raises the question whether physical exercise and inactivity can, in some way, add to the effects of light and dark respectively. Current evidence is not conclusive.

For the first few days in the new time-zone, training sessions should not be all- out efforts. Skills requiring fine coordination are likely to be impaired and this might lead to accidents or injuries if, for example, racket sports players conducted training matches too strenuously. Where a series of tournament engagements is scheduled, it is useful to have at least one friendly match during the initial period, that is before the end of the first week in the overseas country. Subject to these caveats, exercise for racket sports players is recommended also since it helps them psychologically in their preparations for competition.

In practice, therefore, to combine exposure to bright light and exercise, and to combine dim light and relaxation, would seem practicable. However, there is very little evidence to suggest that exercise by itself will alter the speed of adjustment of the body-clock.

To a large extent it might be considered that to adjust as fully as possible to the lifestyle and habits in the new zone would be the best advice. This is not always the case on the first day or so after the flight. Consider a westward flight through eight time-zones. To delay the clock requires bright light at 21:00–03:00 hours body time and its avoidance at 05:00–11:00 hours. By new local time, this becomes equal to 13:00–19:00 hours for bright light and 21:00–03:00 hours for dim light (see Table 2). It can be seen that natural daylight and night would provide this. Consider, by contrast, a flight to the east through eight time-zones. Now light is required 05:00–11:00 hours body time (13:00–19:00 hours local time) and should be avoided 21:00–03:00 hours body time (05:00–11:00 hours local time). That is, morning light for the first day or so would be unhelpful and tend to make

Table 3. The use of bright light to adjust the body-clock after time-zone transitions

* Will advance the body-clock; † will delay the body-clock; # note that this is because the body-clock adjusts to delays more easily than advances.

Travelling racket sports player 105 the clock adjust in the wrong direction (though afternoon and evening light are fine). The timing of exposure to bright light is critical on the first days after the flight. After a couple of days, when partial adjustment has occurred, it is then advised to adjust the timing of the light exposure towards that of the local inhabitants, so that the visitors’ habits become synchronized with those of locals.

Sleep loss itself is unlikely to have a major adverse effect on exercise

performance. In normal conditions the effects of substantial sleep disturbances are more pronounced on complex tasks than on gross measures such as muscle strength (Reilly and Piercy, 1994). Indeed the circadian variation in sports performances was greater than that induced by partial sleep deprivation over three consecutive nights (Sinnerton and Reilly, 1991). Difficulties in sleeping after crossing multiple time-zones are eventually self-correcting but disturbances may last longer following eastward compared to westward flights.

6 Overview

Circadian rhythms should be taken into account when travelling across multiple timezones to compete in racket sports. Deleterious effects of jet-lag will be exacerbated if there are additional environmental stresses, such as heat or altitude, to be encountered. Performance can be adversely affected even when flights are within one country, coast to coast in USA and Australia, for example (Jehue et al., 1993). Whilst jet-lag symptoms persist, even if only periodically during the day, it is recommended that training is light in intensity to reduce possibilities of accidents and injuries occurring. Individuals may be more vulnerable to defeat in the early rounds of tournaments at the hands of home- based players, unless the need for adjustment to the new time-zone is considered in the timetable of the ‘tour’.

7 References

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