The actual reason for sleeping is mainly for our brain. It works by firing synapses which, is how our brain does most of its work. Our brain can only be firing large amounts of synapses for so long before it needs a break. While we sleep, our brain dosnt shut down, it just slows down. Less synapses are fired, and allow it to fire more in a few hours. Thats why we fall asleep, our brain says "hey, give it a rest will ya!"