Jet-lagged mice die younger, researchers said on Monday in a study that suggests that working unusual shifts and flying back and forth across time zones takes a permanent toll on health.

Tests on more than 100 mice showed that old mice forced to live on a confusing schedules of light and darkness, simulating rotating shifts or international travel, died sooner than those on gentler schedules.

Young mice treated in a similar way did just fine, the researchers at the University of Virginia added in a report published in the journal Current Biology.

Gene Block, a professor of biology, and colleague Alec Davidson said they stumbled onto the findings by accident.

Genetically engineered mice in another experiment died when they were put under lights six hours earlier than usual, but no mice died if the light schedule was delayed.

So they tested three groups of mice, with about 30 old mice and 9 young mice in each group.

One group had its light/dark cycle shifted forward by six hours the equivalent of waking people up six hours early every week for eight weeks.

A second group had its schedule shifted back by six hours, and the third group's schedule was unaltered.

They found that 83 per cent of old mice survived under the normal schedule, 68 per cent lived after eight weeks of shifting steadily backward, but fewer than half 47 per cent survived when the lights regularly came on six hours earlier.

When they speeded the schedule up, changing the light schedule every four days, even more mice died.

The mice were not obviously stressed by this their daily levels of a stress hormone called corticosterone did not increase.

"Alternatively, the general frailty of older animals rather than age-related changes in the circadian system may make them less able to tolerate changes in the light schedule," the researchers wrote.

Other studies have shown that hormones associated with wake/sleep cycles, such as melatonin, as well as so-called "clock" genes, can affect ageing and immune system processes.