The endogenous circadian system, in which over 50 physiological and psychological rhythms have now been identified, is known to be affected by many environmental factors. These include local clock hour, light and dark, and temperature, although many ofthe rhythms continue in the absence of such cues, albeit usually with a slightly prolonged periodicity. The environmental factors facilitate entertainment or phasing of the rhythms and are known as synchronisers or 'Zeitgebers' (time givers). Travel across time zones outstrips the ability of synchronisers to entrain rhythms and desynchronisation occurs. This is responsible for the syndrome known as jet lag, as circadian rhythms need a finite period to become re-entrained to local time (usually estimated at about 1 day per time zone crossed). Westward travel is generally considered to be better tolerated than eastward, possibly because the endogenous system, with a natural periodicity in most individuals of about 25 h, is more able to adapt to the longer 'day' encountered during westward flight.
The aetiology of the effects of jet lag—sleep disturbances, disruption of the other body functions such as feeding and bowel habit, general discomfort, and reduced psychomotor efficiency—has been the subject of much investigation. This has largely concentrated on underlying hormonal variations, but for aircrew and business travellers the important changes are those associated with performance levels. Ability at many mental skills, including vigilance, choice reaction time, and simulator performance, rises to a peak during the day between 12.00 and 21.00, with a dip during the afternoon, and then falls to a minimum between 0300 and 0600. Results of memory tests peak in the morning and then fall steadily.
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