Tinkering with the circadian clock, the day-and-night cycle in the physiological processes of all living beings, is rarely a good idea. Poor health and accidents are more common in those who fly frequently or do shift work. Now a new study shows that the direction in which the clock is changed affects the well-being of an individual—at least when the individual is an old mouse.
Researchers led by Gene Block and Alec Davidson of the University of Virginia noticed that, in an earlier experiment, a surprising number of elderly rats died when the daily cycle of light followed by darkness was altered so that the light came six hours earlier. To examine whether there really was a link they conducted a separate experiment using three groups of mice.
Each group contained nine young male mice and about 30 old male mice. The first group was placed on a normal daily cycle of 12 hours of light followed by 12 hours of darkness. The second had its 12 hours of light delayed by six hours once a week, replicating the jetlag experienced after travelling west. The third group had its 12 hours of light advanced by six hours once a week, mimicking eastbound travel. The experiment lasted for eight weeks.
As might be expected, the young mice in each of the three groups fared relatively well; just one died. Of the elderly rodents whose days and nights were unchanged, 17% died. But the number of deaths in the two groups whose day-and-night cycles had been tampered with was far higher. Among the “westbound” group—those whose light cycle was repeatedly delayed—32% died. The death rate in the “eastbound” group, whose light cycle was brought forward, was 53%. The results are published in a recent issue of Current Biology.
To see whether it was the rapidity with which the changes were made that hastened the demise of the old mice, the researchers repeated the experiment but either delayed or advanced the light part of the cycle every four days. Sure enough, in both groups, the mice perished more rapidly. This was particularly the case in the “eastbound” mice, the group whose light cycle had been brought forward.
Exactly why this happened is not yet clear. The researchers investigated the anxiety levels of the mice by measuring a stress hormone called corticosterone in their faeces. The levels remained the same among the elderly mice in all three groups. Therefore the mice did not die because they became stressed by the repeated changes to their body clocks.
Past research has indicated that circadian clocks govern physiological rhythms in many parts of the body and that the different parts can adjust to schedule changes at different rates. Although the experiment did not test the idea (or, for that matter, the effects of an upgrade to the rodent equivalent of business class), the researchers speculate that it may be this lack of synchrony that caused the deaths of the older animals. Whatever the cause, the message for exhausted elderly travellers would appear to be: if you must fly, head west.