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Almost all living organisms have internal biological clocks that are supposed synchronise periods of sleep and wakefulness with external environments.

Scientists from the Max Planck Society  find that mice whose internal biological clocks are deviant, due to some genetic mutations, tend to produce less offspring and have shorter lives than mice whose internal rhythms keep in sync with the 24-hour cycles.

Thus, internal clocks are vital for "coordinating the processes of life" and keeping biological fitness.

With the help of a mutation called tau, which has been noticed to change daily rhythms, scientists checked its influence on biological fitness.

Scientists studied the mice with this mutation that possessed deviant circadian rhythms. They were studied in external environments for more than a year even as they were exposed to a number of natural predators. Every group lived with a number of mice that did not possess the mutation.

Mice without the tau mutation had longer lives and more offspring than those that had the mutation, boosting abnormal biological rhythms. After more than a year in the housing, the tau gene got reduced from 50 percent to 20 percent, which made the researchers believe in the existence of strong natural selection pressures against the tau mutation.

"Our findings highlight the fundamental importance of circadian clocks for the biological fitness of living beings. This has never been shown that clearly," said Michaela Hau, senior author of the study, in a press release.

The findings were published in the Dec. 7 issue of the Proceedings of the National Academy of Sciences.