Sleep is an altered state of consciousness where we have limited interactions with our surroundings and are relatively quiet and still (depending on the stage of sleep). Contrary to our quiet physical state, the brain is very active during sleep, carrying out many important functions. Sleep is essential to every process in the body, affecting our physical and mental functioning the next day, our ability to fight disease and develop immunity, and our metabolism and chronic disease risk. Sleep is truly interdisciplinary because it touches every aspect of health.