This chapter explores the essential function of sleep, detailing the complex neural mechanisms that regulate the sleep-wake cycle. The discussion begins with an overview of circadian rhythms and the stages of sleep. The brain structures, including the hypothalamus, the brainstem, and the thalamus, and the neurochemicals (e.g., orexin, melatonin) involved in maintaining appropriate circadian rhythm and regulating wakefulness and sleep are detailed. The chapter examines a range of major sleep disorders, focusing on the maladaptive components of each. Specific conditions addressed include insomnia, sleep apnea, narcolepsy, and several parasomnias such as restless leg syndrome and somnambulism. For each disorder, the chapter integrates clinical presentation with current neurobiological understanding, highlighting how disruptions in fundamental circuits lead to pathological sleep states and subsequent impairments in daytime function.