"There are a lot of different hormones and other systems in the body that normal under normal situations are internally synchronised," says Shawn Youngstedt. HE studies circadian rhythms at the University of South Carolina in Columbia. The body clock times when those chemicals are released to orchestrate biological activities. Crossing a lot of time zones can jumble these systems. Hormones and the activities they direct may now operate at inappropriate times.
People will recognise this because they develop what is called jet lag. Its symptoms include being tired - and sometimes having an upset stomach.
Consider a runner who has just flown from Denver to Paris. The athlete lands in a city that's 8 hours ahead. In attempting to adjust to this time difference, the runner's hormones must change their routine. For example, the rhythm of cortisol -- a hormone that is influenced by sleep and stress -- might be produced earlier than normal. But the body may delay the release of another hormone. So the rhythms of these two hormones suddenly are misaligned - and the poor traveller feels groggy.
Hormones are just one class of the chemical reactions that maintains life in every cell. Collectively, these reactions are known as the body's metabolism. Those reactions vary a lot over the course of 24 hours, but their schedule on any given day is little different than on any other. And any or all may become perturbed by travel across many time zones.
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