really though, I'd say its because our bodies don't produce enough energy to keep up with our activity, so we have to let our bodies recharge
It is only through food that we get energy. We aren't exactly recharging if that's the case, it's more like having more energy transferred from the brain to other parts of the body.
It is also said that when we sleep, we always dream, and if we don't do so we'd go insane, but why dream as a way to prevent this?