Definition for Sleep
From Biology Forums Dictionary
Sleep is an active process that requires energy and is defined as a state of depressed motor activity and perception that occurs on a cyclical basis. Note that activity is decreased but is not completely absent during sleep.
Several theories have been developed to explain our need for sleep: Sleep may 1) play a restorative role, allowing us to recover from our daily activity, 2) provide us an opportunity to conserve energy, 3) allow us to dream, providing time for mental practicing, 4) facilitate long-term memory storage, and 5) provide a time for the immune system to function optimally.
