Sleep pressure / two-process model

The two-process model, proposed by Alexander Borbély in 1982, describes sleep-wake regulation as the interaction of two independent processes: Process S (homeostatic sleep pressure), which accumulates as adenosine and other somnogens build up during waking and dissipates during sleep, and Process C (the circadian signal), generated by the suprachiasmatic nucleus and imposing a roughly 24-hour oscillation in alerting drive that opposes increasing sleep pressure toward evening to maintain sustained wakefulness. Sleep occurs when Process S exceeds the circadian alerting threshold. The model successfully explains phenomena such as the post-lunch dip, rebound deep sleep after sleep deprivation, and the sharp morning wake boundary, and remains the dominant framework for circadian and sleep medicine research.