Abstract: Individuals have biological rhythms that are coordinated for optimal functioning. The cycle of sleep and wakefulness is regulated by two main processes: the circadian regulation (process C) and the homeostatic regulation (process S). Sleep stages are regulated by ultradian cycles. Recent research has brought light to neurochemical and molecular interactions and complex organization of the networks involved in the timing of sleep and wakefulness. In this chapter we will give an overview of these processes and how they interact to synchronize sleep-wake cycles.