ABSTRACT

Since the discovery of the ascending reticular activating system more than sixty years ago, the anatomy, electrophysiology, and neurochemistry of the neuronal networks involved in generating and maintaining wakefulness, that is, the activating

systems have been characterized in detail. Furthermore, the neural areas critically involved in the generation and maintenance of rapid eye movement (REM) and nonREM (NREM) sleeps, which are called the hypnogenic systems, have also been delineated. The activating and hypnogenic systems deeply interact in order to induce the sleep/wakefulness cycle. These systems are modulated by the suprachiasmatic nucleus (SCN), the circadian rhythm pacemaker, as well as by various somnogenic substances such as adenosine and melatonin.