Features

Watching your clock

20 Feb 2007 by business traveller

Jet lag is a fact of life for most frequent flyers. Like hangovers or the common cold, there is no cure, just plenty of short-term remedies. As business travellers we think nothing of squeezing in half a dozen long-haul trips a month, with the result that many of us live under a semi-permanent cloud of jet lag.

Recent research suggests that in the long run this lifestyle might not be in our best interests. Scientists at the University of Virginia looked at the survival of old and young mice which had been subjected to regular shifts in their light-dark cycle – similar to the conditions we experience when frequently crossing time zones or working night shifts.

The results were surprising. The old mice experiencing "jet lag" had a much higher mortality rate than old mice on the normal daylight schedule over the same eight-week period. They also had a significantly higher death rate than the young jet-lagged mice.

Even more surprising was that the old mice were more likely to survive (68 per cent survival rate) if their light cycle was delayed by six hours – equivalent to crossing six time zones west – than if it was advanced (47 per cent), similar to flying six hours east.

To check that stress did not kill the mice, the scientists measured their levels of the stress hormone cortisol throughout the experiment and found no major changes. Instead, they suggested that age-related changes in the animals' circadian clocks might have made them more vulnerable to the disruption of jet lag.

The researchers concluded: "Whatever the precise mechanism, the dramatic differences in morbidity associated with phase advances of the biological clock raise important issues about the safety of counter-clockwise rotating shift work and the potential long-term health consequences for airline crews regularly crossing time zones."

Humans, like mice, have an internal circadian rhythm which governs our sleeping and waking cycle and is set by external cues like daylight and darkness, eating and socialising. But the body clock controls much more than our sleeping patterns. Dr Andrew Coogan, lecturer in neuroscience at the University of Wales Swansea, says: "Circadian rhythms are increasingly being recognised as important in a number of processes – mice with mutations in genes associated with the circadian clock show abnormalities such as obesity and metabolic syndrome, increased tumours and shortened lifespan.

"Also it is well known that there are strong circadian rhythms in the occurrence of heart attacks and strokes in humans, which tend to occur in the morning."

It is not fully understood what role the body clock plays in these processes, and for the two groups most commonly studied for signs of circadian rhythm disruption – shift workers and airline staff – it is still not clear what the long-term effects of this disruption might be.

Night-shift workers have been found to experience a higher incidence of gastrointestinal diseases and heart disease than the general population, and some studies have found increased incidence of breast cancer among female flight attendants – although other factors have also been suggested, such as radiation at altitude and a higher tendency of flight crew to smoke.

There is another reason why the results might not be clear, says Dr Chris Idzikowski, director of the Edinburgh Sleep Centre and sleep consultant for British Airways. "In the 1960s and 1970s there were a lot of studies of shift workers, but the problem with these sorts of jobs is that people tend to 'self select' – that is, those who can tolerate [the disruption to their light–dark schedules] stay in the job," he explains. "The mice [in the study] had no such choice – perhaps that gives a truer estimate."

The same rule may apply to business travellers: you either adapt to a life of travelling or find another job. A recent survey of 1,002 business travellers commissioned by British Airways found that older executives reported fewer problems coping with jet lag than younger managers – Idzikowski suggests that those who cannot handle the peripatetic lifestyle may have dropped out before reaching senior management level.
In general, though, our sleep tends to become poorer as we get older. "You get less deep sleep when you're older so you're more vulnerable to waking up," says Idzikowski. This tendency means that elderly people are likely to find jet lag harder to cope with.

But everyone is different, and there is also a strong genetic basis that determines how we cope with jet lag. Research has shown that it tends to be "night owls" – those who prefer to go to bed late and rise later – who cope best with the disruption. "Morning larks", on the other hand, are early risers who prefer early nights and tend to have more rigid sleep routines and thus find it harder to take naps while travelling to counteract the effects of jet lag.

As for whether travelling east is more harmful, many travellers report that it takes longer to recover from jet lag after flying east, perhaps because our body clocks naturally run fractionally longer than 24 hours, making it easier to lengthen your day than to shorten it. Unlike the mice, however, people travel one way and then back, rather than repeatedly eastwards, so don't ban all eastward travel just yet.

Ultimately, says Coogan, "it is highly unlikely that chronic jet lag will do you any good", but he emphasises there is not yet enough evidence to understand its impact. In the meantime, older business travellers can take comfort from the fact that if you are still going strong, you may be one of the jet lag "elite" with good genes for coping with the disruption – or you may simply have learned to live with it.

It should also be a comfort that the old mice in the study were, in fact, ancient mice, equivalent to humans in their eighties and nineties and, says Coogan, "I cannot imagine that there are too many very frequent long-haul passengers in this age range." Sometimes it just feels that way.

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