Sleep is essential to a healthy human being. It is among the basic necessities of life, located at the bottom of Maslow's Hierarchy of Need. It is a dynamic activity, necessary to maintain mood, memory and cognitive performance. Sleep disorders are strongly associated with the development of acute and chronic medical conditions. This article traces the history of sleep and then discusses the physiology of sleep. In this discussion, two main sleep types--REM (Rapid Eye Movement) sleep and NREM (Non-REM) sleep--and the four stages of Non-REM sleep are described. Other topics covered include sleep and aging, sleep and children, and how to improve sleep quality.