There are two ways by which brains functions are classified. Once is the conscious part of the brain, and the other is the sub-conscious part of the brain, when a person is awake, both the conscious and sub-conscious are active. When a person is dead, both are in active. When a person is sleeping, the conscious brain is at rest, while the sub-conscious mind is active and has been the cause of all dreams and intuitions. Nature is always symmetric. Hence this leads us to predict that there will be one more state, where the sub-conscious mind is at rest and the conscious mind is awake. This is the state which consumes the least amount of energy and our ancestors where pioneers in controlling the sub-conscious mind and this state is also called meditation. In sleep two kinds of sleep - rapid eye movement sleep (REM) 25%, NREM (non-REM sleep) 75%. REM sleep you get dreams, normally you get REM after midnight. The store houses (ATP, Phosphate, Glycogen reserves) are replenished during the NREM sleep. It prepares the body for the next day activity. REM sleep prepares the mind. During NREM Sleep brain is prepared keeping all the important thing stored and wiping off the unwanted things.