Chronotype asymmetry arises from stochastic sleep homeostasis under circadian entrainment

Study ObjectivesChronotype, typically defined by the mid-sleep phase on free days, is often interpreted as a direct proxy for circadian phase. However, a fundamental paradox challenges this assumption: worldwide chronotype distributions show a heavy right skew, whereas circadian period distributions show a slight left skew. We investigated whether fast, noisy cortical processes in homeostatic sleep pressure dynamics could resolve this paradox. MethodsDrawing upon the classical two-process model of sleep regulation, we simulated how interindividual differences in sleep homeostasis (Process S), combined with realistic circadian period distributions (Process C), interact to shape chronotype variability. We introduce stochasticity into sleep pressure build-up to test whether homeostatic variance generates the empirically observed right-skew in chronotype distributions. ResultsExploring the parameter space reveals that stable sleep-wake cycles are constrained within physiological limits of entrainment in an Arnold tongue-like structure. While our simulations confirm a general positive correlation between intrinsic period and chronotype, the heavy right tail of the chronotype distribution arises from interindividual variability in homeostatic sleep dynamics operating near the unstable boundaries of entrainment. This mechanism preserves the individual-level correlation but inverts the population-level skew from left to right. ConclusionsBehavioral chronotype emerges from the nonlinear interaction between fluctuating homeostatic sleep pressure and a hierarchically entrained circadian system. We caution against inferring the intrinsic circadian period solely from chronotype, particularly in clinical contexts where accurate assessment of circadian rhythmicity is critical for targeted therapies.