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“Step Aside Circadian Rhythm: Scientists Discover Independent 12-hour Clock”

If our sleep cycles weren’t complicated enough, balancing work, family, sleep, and countless else, it’s just gotten more complicated.  Our circadian rhythm, the standard 24-hour cycle, is not alone in regulating our bodies.

Sleep observations, circadian oscillations, and other related sleep phenomena have been studied since the 4th century, the time of Alexander the Great, and has been continued to be researched.  In the 1970s, scientists tied those rhythms to genes in Drosophilia.  In 1994, the circadian rhythm was genetically identified in mammals.  Research has continued in sleep and body cycles, however it took more than two decades for the 12-hour clock to be uncovered in mammals.

Bert O’Malley and his team at Baylor College of Medicine unveiled the distinct 12-hour clock in mammals that operates independently from the standard 24-hour circadian rhythm.  The 12-hour clock has been observed in sea creatures, which is believed to correspond with the tide, but it has never been observed previously in mammals.  A possible argument is that as mammals evolved from the sea millions of years ago, they held on to the 12-hour clock.

The 12-hour clock might have been overlooked as background noise to the 24-hour circadian cycle, which is more distinct.  Regardless of the reasons, these rhythms have tremendous physiological effects: they can alter how alert and clearly we think, how we respond to stress, and how we metabolize and respond the medications.  By further understanding this additional body rhythm and the respective gene expression, we may be able to better understand our bodies’ physiology.


  • O’Malley and his team looked at the gene-expression of over 18,000 mouse liver genes.

    • Over 3,500 genes with 12-hour rhythms were independent of the mouse’s circadian rhythm

  • Researchers analyzed RNA sequencing data from mice with and without the BMAL1, the master gene regulating the circadian clock.

    • 91 genes were studied

    • 44 genes were shown to have 12-hour rhythms in the absence of BMAL1

  • Stress Gene XBP1, was previously associated with the circadian clock, however in O’Malley’s studies, the gene was shown to regulate the 12-hour clock.

  • Data was compared with crustacean Eurydice pulchra, nematode elegans, and rhythms coming to the conclusion that the 12-hour clock evolved separately

    • Both pulchra and C. elegans have 12-hour clocks. In the crustacean, the clock is coupled with the tides.  In the nematode, it’s affiliated with feeding and stress


Kim Buck | Lead Recruiter, Account Manager


  1. Zu et al., “A cell-autonomous mammalian 12 hr clock coordinates metabolic and stress rhythms,” Cell Metabolism, doi:10.1016/j.cmet.2017.05.004, 2017.

Yeager, Ashley (2017 June 6).  Mammals may have a 12-hour clock.  The Scientist.