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Thursday, 27 April, 2000, 18:01 GMT 19:01 UK
Jetlag clues uncovered

Long-haul travellers often suffer from jet lag
Experiments involving rats suggest that the body's liver, muscles and lungs each have their own internal 'clocks'.

And this could explain why jetlagged travellers, or workers changing shift patterns, can feel genuine aches and illnesses because of the time differences.

The rat study, carried out by scientists in the US and Japan, and published in Science magazine, found that the internal 'circadian' rhythms of muscle, liver and lung can be thrown out of synchronisation with the main body circadian cycle.

The researchers linked a fluorescing firefly protein to a key circadian clock gene in genetically modified rats, causing the rats' cells to emit light when the clock gene was activated.

Jet lag induced

The researchers then induced jet lag in the rats by fast-forwarding or delaying the light/dark cycles in the rats' environment and then studied the circadian activity in different tissues trying adjust to the change.

The clock in the brain re-set itself within just one revolution of the circadian cycle, while the clocks in the muscle and lung took six cycles to re-set, and the liver took more than 16.

It can cope with the subtle seasonal changes which reduce or increase the number of daylight hours - but cannot deal with sudden changes like those caused by transatlantic travel.

So even if the main cycle has been reset to a different time zone, the liver may still be functioning out of step.

However, it is not certain how accurately circadian rhythms in rats reflect those in humans.

As well as fatigue, jetlagged people can often have upset stomachs and aching muscles.

The researchers estimated that up to 20% of the US workforce is exposed to these abrupt day to night or night to day shift changes.

Dr Mary Morrell, a lecturer in sleep physiology at the National Heart and Lung Institute in London, said that passengers flying long-haul across time zones often complained of gastric symptoms.

"There has been a lot of discussion about whether these separate parts of the body have their own circadian cycles, and this is more evidence to support it."

She added that the trial would be extremely difficult to replicate in humans, as it would involve keeping them in almost unethical conditions.

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