Airline crew and passengers who frequently fly between several time zones face a number of health problems including disruptions in a woman's menstrual cycle and even short-term psychiatric disturbances, researchers from the UK warn in a report.

There seems to be no getting use to long-haul flights, according to researchers who report that flight crews who regularly take long journeys are not protected from the effects of jet lag such as poor and interrupted sleep, mood changes, irritability, stomach problems, and decreased brain power.

Jet lag from crossing several time zones also causes a dip in an athlete's performance, note Jim Waterhouse and two associates from the Research Institute for Sport and Exercise Sciences at Liverpool John Moores University.

Jet lag is worse for older travellers, and its severity increases with the number of time zones crossed. "If the journey crosses fewer than about three time zones, then jet lag is unlikely to be a major difficulty for most people," the authors write.

The direction of travel also matters, Waterhouse and colleagues say, with flights to the east bringing more jet lag than flights to the west.

So how long will it last? Jet lag caused by eastbound flights lasts for several days "roughly equal to two-thirds of the number of time zones crossed, and about half the number of time zones crossed for westward flights," Waterhouse and colleagues report.

Currently, there is no cure for jet lag, but there are some things frequent long-haul travellers can do to try to lessen the impact of jet lag upon arrival. For journeys that cross more than three time zones, travellers can help the body clock adjust by deliberately seeking or avoiding sunlight at the new destination, the investigators offer.

Trying to maintain alertness during the day at the new destination by exercising and/or drinking caffeinated beverages may also help. The jury is still out on the value of taking the hormone melatonin to curb jet lag, the authors say. Melatonin is secreted during sleep and has been implicated in jet lag, but Waterhouse and colleagues don't advise using melatonin until more research is conducted.

What's needed, they conclude, is a "more detailed understanding of the molecular changes associated with time zone changes ... with a view to developing drugs to promote clock adjustment and further assessments of new sleep-promoting and alertness-promoting drugs."