Fig. 3: A neuronal mutual inhibition model and its relation to the two-process model.

a Neuronal structure underpinning sleep-wake regulation, based on the conceptual framework of Saper, Scammell & Lu²² as framed by Phillips & Robinson²³. (In this model the impact of the suprachiasmatic nucleus (SCN) is primarily by variation of inhibition of the sleep drive. Based on physiology, it is more likely that the SCN primarily activates the wake promoting part of the system during most of the 24 h day, with only small contributions to sleep drive late in the biological night⁴¹). b–e Simulation of the Phillips-Robinson model for standard parameter values. b Homeostatic sleep pressure increases during wake and decreases during sleep. c Sinusoidal representation of the circadian rhythm from the SCN entrained to 24 h. d Firing rate of wake active neurons (high during wake and low during sleep). e Firing rate of sleep active neurons (high during sleep and low during wake). f Hysteretic representation of the Phillips-Robinson model. In this representation, there is a folded surface which results from the mutual inhibition of the wake active and sleep active neuronal populations. The time dependent sleep drive results in a trajectory (blue/red) that evolves over the surface, switching between the wake (upper) surface to the sleep (lower) surface at the folds (tipping points) which are indicated by the black circles on the black line. g The Phillips-Robinson model in the form of the two-process model. The homeostatic sleep pressure (blue/red curve) increases during wake and decreases during sleep and is colour-coded as in panel f. The mean level of the upper (lower) threshold is given by the value of the drive at the upper (lower) fold in panel f. The oscillation of the thresholds is given by the circadian inhibitory input to the sleep drive. The hysteretic structure suggests that it is the upper threshold that is of prime relevance during wake, and the lower threshold which is of prime relevance during sleep, we have therefore drawn the relevant `active' threshold in dark grey and the 'inactive' threshold in light grey.