Jetlag - or working irregular shifts - damages health, a US study of elderly mice has suggested.

Animals who were subjected to changes like those experienced by humans with jetlag or who work unusual shift patterns, died earlier than others.

Writing in Current Biology, the researchers say it raises concerns for humans affected by time disruption.

But a UK expert said, while the study was interesting, frequent-fliers and shift-workers should not panic.

Sleep deprived

The body's physiological reaction to the circadian rhythm - the natural cycle of light and dark - is thought to be complex, but the effects on the body are not fully understood.

The team, from the University of Virginia, compared how old and young mice were affected by changes to the usual balance of "day" and "night".

In one regimen, the mice's clocks were "put forward" by six hours once a week - the equivalent of the time difference between the UK and Dhaka in Bangladesh - so they had less time in the dark.

Other mice experienced a six-hour backward shift - and therefore more time in the dark - which would equate to the difference between the UK and Chicago.

Separate groups of young and old mice had normal cycles.

Younger animals appeared unaffected by alterations to their schedule.

But only 47% of the older mice whose "nights" were shortened survived, compared with 68% of those whose nocturnal time was lengthened and 83% of those who remained on a normal schedule.

Chronic stress - which has been cited as a mechanism for causing ill-health in those with disrupted schedules and which can be measured through daily corticosterone levels - did not increase in any of the old mice.

The researchers suggest the cause of increased mortality in the mice could be linked to sleep deprivation or immune-system disruption.

They also suggest that age may alter how the circadian system works, or that their elderly mice's general frailty might mean they are less able to tolerate changes in light cycles.

24-hour society

Writing in Current Biology, the team led by Dr Gene Block, said: "Whatever the precise mechanism, this raises important issues about the safety of counter-clockwise rotating shift work [where people's shift patterns are altered causing them to have less sleep] and the potential long-term health consequences for airline crews regularly crossing time zones."

Dr Malcolm von Schantz, senior lecturer in physiology and biochemistry at the University of Surrey, told the BBC: "This is an interesting area which needs research because it's something that's happening more and more.

"There is an increasing amount of cross-time zone travel, and of a '24-hour' society.

"We need to carry out more research, on animals and humans, so that we can properly assess the risk."

He added: "People shouldn't panic, and shouldn't stop travelling or working shifts because of these findings.

"However it's a reminder that, just because humans can do something doesn't mean it's necessarily safe in the long-term.

"And it's important to remember this work was carried out on mice, who are nocturnal animals, while humans are diurnal."