Emergent Multidien Cycles From Partial Circadian Synchrony

Over the past decades, chronobiology has attracted great attention thanks to the elucidation of the molecular mechanisms underpinning the circadian cycle. Now, growing evidence suggests that cycles longer than circadian, so-called "multidien" cycles, are of crucial importance in physiological fluctuations spanning multiple days with repercussions in health and disease. Unlike circadian clocks, multidien cycles may not be genetically encoded, given their heterogeneity within and across individuals and systems. Here, we propose that multidien cycles may be generated by the interaction between partially coupled circadian oscillators. To demonstrate this possibility theoretically, we use a ring model of coupled circadian oscillators and study how synchrony within this network evolves over time. We found that a free-running, about-weekly period robustly emerges from the network's dynamics. A range of additional multidien cycles resulted from subtle variations in the coupling parameters within the network with periodicities reminiscent of those observed across different species. Thus, our model of emergent multidien cycles from partial circadian synchrony constitutes a credible hypothesis for explaining the timing of a myriad of events on the scale of weeks and months in health and disease.