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Risky trip?
A disruption of day/night cycles harms older mice and could pose a danger to humans.

Jet Setting Is For the Young

As most jet setters know, life in the fast lane exacts a high price. Any departure from usual day/night routine can play havoc with biological clocks and lead to chronic jetlag. Now, a new study in mice suggests that jetlag--and analogous conditions suffered by shift workers and others--might be more than just a nuisance but a serious danger to health.

There have been surprisingly few studies of the dangers of chronic jetlag to humans. What little is known suggests the condition can increase the risk of cancer, peptic ulcers, and sleep disorders. These findings are of great concern to specialists in occupational medicine, because a growing number of shift workers--up to 75% according to a recent European Union study--are working shifts outside of the normal 8 AM to 5 PM work day. Moreover, the risk of illness from jetlag appears to be greater with advancing age.

To investigate the role of aging in jetlag, biologist Gene Block of the University of Virginia at Charlottesville and his colleagues exposed both young mice (8-12 months) and old mice (27-31 months) to light-cycle shifts over 8 weeks. One group made up of 9 young and 30 old mice had their day/night cycle moved forward 6 hours every week, while another group of similarly aged mice had its day/night cycle shifted 6 hours backwards every week. A control group was exposed to a normal cycle of 12 hours of light and 12 hours of darkness. While only 17% of the old control mice died during the study period, 53% of the old mice whose cycles were moved forward died and 32% of the old mice whose cycles were moved backward died. Only one young mouse died during the experiment.

The study sounds an important warning about the possible dangers of jetlag, especially among older people, the team concludes in tomorrow's issue of Current Biology. Francis Lévi, a researcher who studies cancer risk and biological rhythms at the Paul Brousse Hospital in Villejuif, France, says that the new study is consistent with several epidemiological studies in humans. "There is an increased risk of breast or colorectal cancer in women exposed to chronic shift work for longer than 15 years," Lévi says, adding that more research on the role of jet lag in illness should be a high priority.

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