Abstract
This chapter describes how sleep is regulated by a process that tracks sleep–wake history and a circadian process. It focusses on human studies, but it is emphasized that many principles of this twoprocess model for the maintenance of homeostasis also apply to animals. It reviews the empirical fundamentals, conceptual representation, and a few relevant mechanistic details of this twoprocess system. It explains how the separate contributions of sleep–wake history and circadian rhythmicity can be assessed in protocols such as the forced desynchrony and constant routine protocol. It summarizes how sleep timing and structure as well as brain function during wakefulness are modulated by these two processes. It also reviews how environmental factors such as light and social factors such as work schedules influence the timing of sleep and circadian rhythms. Implications for the understanding of sleep timing disorders are discussed.