Orexin A and orexin B are hypothalamic neuropeptides that were discovered in 1998. Several studies suggested that orexin deficiency causes narcolepsy in humans and other mammalian species, highlighting roles of this hypothalamic neuropeptide in the regulation of sleep and wakefulness. The orexin system regulates sleep and wakefulness through interactions with systems that regulate emotion, reward and energy homeostasis. This system regulates sleep and wakefulness to occur at appropriate times that are in accordance with our internal and external environments. Recent findings have brought about the possibility of novel therapies targeting orexin system for sleep disorder including insomnia and narcolepsy-cataplexy. In this review, I will discuss the current understanding of the integrative physiology and clinical perspectives of the orexin system.