Human data at odds and in confirmation of the two-process model of sleep regulation-a perspective.

This perspective summarises human data collected in experiments conducted over a 40-year period designed to test the two-process model of sleep regulation in which sleep is regulated through the interaction of circadian rhythmicity and sleep homoeostasis. The perspective highlights findings in accordance with and at odds with predictions and assumptions of the model. Implications for understanding how circadian and sleep-wake dependent processes interact to regulate homoeostasis of brain function are discussed.