Jet lag could be a killer

Researchers in the U.S. have found that jet lag or rotating shift work shortens the lifespan in mice and they say it raises concerns for humans affected by time disruption.

A team led by Dr. Gene Block and Dr. Alec Davidson from the University of Virginia, looked at how old and young mice were affected by changes to the usual balance of "day" and "night" over an eight-week period.

They found that aged mice undergoing weekly light-cycle shifts similar to those that humans experience with jet lag or rotating shift work, experienced significantly higher death rates than did old mice kept on a normal daylight schedule.

The findings may not be a such a revelation to those who regularly experience the exhaustion which results from flying from one side of the world to the other but it does provide new insight into how the disruption of circadian rhythms, the natural cycle of light and dark, can impact on well-being and physiology, and how those impacts might change with age.

The researchers were prompted to take a closer look at jet lag and mortality after an earlier but unrelated study found a surprising number of old genetically altered rats exposed to a six-hour advance in their light cycle died after the experience.

For the new study the researchers examined the mortality link in earnest by looking at how young mice and old mice did when subjected to two different types of light-cycle shifts.

In one regimen the mice experienced a six-hour forward shift once a week, so they had less time in the dark; in the other, mice experienced a six-hour backward shift so had more time in the dark, while a "control" group of young and old mice did not experience any schedule shifts.

It was found that the young mice generally survived well under the various conditions but the light-cycle shifts had a marked effect on the survivorship of the old mice; while 83% of old mice survived under the normal schedule, 68% survived under the backward-shift regimen and only 47% survived under the forward-shift regimen.

Previous research has also linked changes in light schedule with death in animals, but these results indicate that there may be a different effect on mortality depending on the direction of the schedule shift; forward or backward.

The researchers also found that chronic stress, measured by daily corticosterone levels, did not increase in the old mice experiencing the light-cycle shifts.

They say the underlying cause of the increased mortality is not yet clear, but could involve sleep deprivation or immune-system disruption.

Scientists believe the body's physiological reaction to time change may be a complex one and previous research has indicated that circadian clocks govern physiological rhythms in a great variety of tissues in the body, and that different aspects of the physiological clock can adjust to schedule changes at different rates.

The researchers speculate that the internal lack of synchrony among different physiological oscillations may have serious health consequences that are exacerbated in aged animals.

Experts say as there is an increasing amount of cross-time zone travel, and of a '24-hour' society, more research, on animals and humans, is needed in order that the risk can be properly assessed.

The research is published in the November 7th issue of the journal Current Biology.