Deep sleep is a stage of sleep in which a person is completely unconscious and unaware of their surroundings. It is the deepest stage of sleep and is usually the longest, lasting between two and four hours. During deep sleep, the body repairs and regenerates itself and the brain is inactive.

REM sleep (or Rapid Eye Movement sleep) is the stage of sleep characterized by dreaming and increased brain activity. This stage of sleep is much lighter than deep sleep, and people are more easily aroused from it. Also unlike deep sleep, during REM sleep the body’s muscles are relaxed and the eyes move rapidly from side to side.