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New Delhi, Nov. 7: Repeated tampering with biological clocks, an effect that occurs through chronic jet lag and rotating shift work, appears to shorten the life span of aged mice, a new study has shown.

Scientists in the US have found that aged mice exposed to weekly shifts in day-night cycles experienced higher death rates than did aged mice kept on a normal cycle during eight weeks of experiments.

Biologist Gene Block and his colleagues at the University of Virginia exposed groups of young and aged mice to either normal day-night cycles or to changing cycles that would mimic what the body’s biological clock would experience through travel across time zones or through rotating shift-work cycles.

In the journal Current Biology, the scientists said a six-hour advance of the light cycle — the same as shifting the “day” six hours ahead — hastened the death of aged mice.

While young mice survived day-night cycles, the death rate in aged mice was three times higher in the group exposed to the advanced light cycle compared to the group exposed to the normal cycle.

“I’m not surprised by these results,” said Vijay Sharma, a scientist who specialises in studying biological clocks of organisms at the Jawaharlal Nehru Centre for Advanced Scientific Research in Bangalore. “As organisms grow older, their capacity to adjust to rapidly changing day-night cycles might weaken.”

Many previous studies have shown that rotating-shift work and chronic jet lag can lead to a desynchronisation of the biological clock with the environment. It takes some time for this clock to synchronise itself with a new day-night cycle, although this period can differ from one individual to another.

But researchers say harmful effects of desynchronisation of the body clock with the day-night cycle are expected to surface only in case of persistent desynchronisation.

“Most people on fixed shifts will have no problem because their internal clocks have already synchronised themselves with the day-night cycle that they follow in their work schedules,” Sharma said.