The quest to learn about the need, organization and the functions of sleep has led to basic understanding of physiology of human sleep. Just like wakefulness, different stages of sleep exert unique influence on organ systems of the body. The amount and quality of sleep seems to be in part, controlled by the competing endogenous rhythms. The autonomic system demonstrates characteristic changes with each sleep cycle of non rapid eye movement and rapid eye movement sleep. Cardiovascular physiology provides evidence of “rest and relaxation” during most of the sleep. While behavioral and wakefulness drive to breathe ceases during sleep, respiratory mechanics and ventilatory responses change predictably. Sleep influences most hormone secretion and their actions. Thermoregulation is linked to the sleep wake cycle. Thorough understanding of sleep physiology not only facilitates the approach to diagnosis and management of clinical sleep disorders but also serves to stimulate inquiry, discovery and innovation.