Deep Sleep refers to a stage of sleep that is characterized by slow brain waves, low muscle tone, and reduced consciousness. It is one of three main stages of sleep (the others being light sleep and REM sleep), and typically lasts for around 30 minutes during each cycle. During deep sleep, the body repairs itself, builds bone and muscle mass, strengthens the immune system, and consolidates memories from the day. It is essential to get enough deep sleep in order to feel rested and refreshed upon waking up.