Circadian regulation dominates homeostatic control of sleep length and prior wake length in humans

Abstract

During prolonged temporal isolation in caves or windowless rooms, human subjects often develop complicated sleep-wake patterns. Seeking lawful structure in these patterns, we have reanalyzed the spontaneous timing of 359 sleep-wake cycles recorded from 15 internally desynchronized human subjects. The observed sleep-wake patterns obey a simple rule: The phase of the circadian temperature rhythm at bedtime determines the lengths of both prior wake (α) and subsequent sleep (ρ). From this rule we derive an average α:ρ relationship that depends on circadian phase. The relationship reconciles the established negative α:ρ correlation observed in synchronized subjects with the positive α:ρ correlation found in desynchronized subjects. Our most surprising result concerns the residual deviations of α and ρ from their circadian phase-adjusted mean values. We report that there is no significant positive correlation between the residuals of α and ρ, contrary to the prediction of restorative models of sleep duration. Our findings illuminate the mechanisms underlying sleep regulation and provide much-needed tests of mathematical models of the sleep-wake cycle.