Modeling the circadian substructure: coupled oscillators and entrainment

Many circadian rhythms exhibit two peaks, but most mathematical models of circadian rhythms have not examined this issue. The authors simulated the bimodal circadian substructure with a system of two coupled oscillators where one oscillator´s natural frequency was in the circadian range (20-30 hr.) and the second oscillator´s period was roughly half of the circadian period (e.g. 12.5 hr). The circadian part of this system could be entrained by external stimuli, but the high-frequency second oscillator was only weakly affected by entraining stimuli. Important features of actual data that were simulated by this system were bimodal peaks, with the secondary peak moving progressively through the main peak, and low-frequency fluctuations, below the frequency of either oscillator.<>