WASHINGTON, March 29 (Xinhua) -- Menstrual cycle problems and transient psychotic disorders are just some of the dangers faced by air crew who repeatedly work long haul routes, according to a study reported Thursday by the American Association for the Advancement of Science.

    The regularity of such journeys offers air crew no protection from the effects of jet lag, and can lead to decreased brain power and increased occurrence of major affective disorders.

    Jim Waterhouse, from Liverpool John Moores University, UK, and colleagues also looked at the effects of jet lag on athletes -- finding impaired performance in training after crossing multiple time zones.

    The symptoms of jet lag are diverse and are caused by inappropriate timing of the body clock in the new time zone. Sufferers report poor and interrupted sleep, early or delayed sleep depending on the direction of travel, mood changes, headaches, irritability and gastrointestinal difficulties.

    The crossing of time zones disrupts the body's daily cycles (termed circadian rhythms). Sleep is badly affected because one such circadian rhythm -- the body's core temperature -- takes time to adjust. The hormone melatonin -- usually secreted during sleep -- is also implicated in jet lag symptoms, according to the researchers.

    For longer journeys (of over three time zones for more than three days), deliberately seeking or avoiding light at the new destination helps the body clock adjust, as does maintaining daytime alertness at the new destination by taking exercise and/or using regular stimulants such as caffeine. But they caution against the use of external melatonin until significant field studies have been completed

    The researchers conclude in their article published in this week's issue of The Lancet that "a more detailed understanding of the molecular changes associated with time zone changes is needed,with a view to developing drugs to promote clock adjustment, and further assessments of new sleep-promoting and alertness promoting drugs."