Sleep and wakefulness are greatly influenced by various physiological and psychological factors. However, the neuronal elements responsible for organizing sleep-wake behavior in response to these factors are largely unknown. We have discovered that (1) neurotensin neurons in the lateral hypothalamus may play a crucial role in modulating sleep-wake states, and (2) neurons containing melanin-concentrating hormone in the posterior lateral hypothalamus play an integral role in rapid eye movement sleep regulation.