I am a disaster zone without sleep, my emotions are looser and thinking shallower. I need sleep so that the mental books and notes are stacked on the shelf neatly in order rather than left scattered on the table (if you know what I mean.)

I read that humans started evolving much more rapidly after dogs were domesticated and employed to protect and warn sleeping humans of approaching wild animals. All of sudden we could catch more sleep.

From a simple view, it is to relax our bodies. The amount may vary from person to person. Naps after lunch can also help reduce sleep required at night (depends).

If you are in some physical work, sleep will come naturally. For work that requires mental alertness, sleep will be less easy.

Its like asking “why cant we run the car all the time, if we keep adding fuel to it?”

Sleep will help rejuvenate the body (brain too).