The evolution of animals is a consequence of selective specialization of cells, tissues, functional systems, and behavior. The objective of all life is successful reproduction and maintenance of the species. In order to accomplish this, all animals have evolved a division of behavior into two fundamental behavioral states: one characterized by the elaboration of adaptive behavior (activity) and the other by rest and behavioral quiescence (rest). In mammals, these states are designated wake and sleep. Activity and rest, wake and sleep, occur in precise 24-h cycles that have evolved as an adaptation to the solar cycle of light and dark. These cycles are referred to as circadian rhythms. Over the last 40 years, we have gained detailed knowledge about the neurobiology of both wake and sleep and the circadian control of their timing. We now know that circadian rhythms are present in cells throughout the body. In the brain, a small group of hypothalamic nerve cells, the suprachiasmatic nucleus (SCN), functions as a master circadian pacemaker controlling the timing of the sleep-wake cycle and coordinating this with circadian rhythms in other brain areas and other tissues to enhance behavioral adaptation. In this review, I will summarize our current understanding of the organization and function of the circadian timing system and its role in the regulation of brain mechanisms of sleep and wake.