No one is really sure…some believe that sleep gives the body a chance to recuperate from the day’s activities but in reality, the amount of energy saved by sleeping for even eight hours is miniscule – about 50 kCal, the same amount of energy in a piece of toast.
But we do have to sleep because it is essential to maintaining normal levels of cognitive skills such as speech, memory, innovative and flexible thinking. In other words, sleep plays a significant role in brain development. Sleep time is when our brain processes a lot of stuff and, at least the first initial bit of sleeping, allows us to move things from short term to long term memory!! Basically our brains need to sleep if we are going to be able to function properly but actually it is active when we are asleep so it must be using this time to do a LOT of stuff that we just don’t understand yet! 🙂