During your lifetime, you will likely spend 25 years or more sleeping. But why? What is the purpose of sleep? Surprisingly, scientist don't know for sure. Scientists used to think we "turned our brains off" when we went to sleep. Sleep researchers now know, however, that our brains are very active when we sleep. Some scientists think we sleep in order to replenish brain cells. Other scientists think that sleep helps the body to grow and relieve stress. Whatever the reason, we know that it is important to get enough sleep.