All mammals sleep. Many birds, reptiles and fish do, too. Sleep is a naturally occurring cycle where sensory input is diminished and the body quiets down. Consciousness is reduced, but dreaming occurs, at least in humans. Much is not known about sleep but it is the intense focus of scientific study. It is believed that sleep helps rejuvenate the body. Sleep is divided into two main types, rapid eye movement (REM) sleep and non-rapid eye movement (NREM) sleep. During NREM, the sleeper experiences three stages, characterized by alpha, theta and delta waves. Most vivid and lucid dreaming occurs during REM sleep.