I think the sleep does more than just this.

I have experienced that I have solutions to my questions/problems. Maybe partly the emotions play a role here to subside during sleep and reanalyze every aspect of the thought process and put them in order. Because when I wake up in the morning, I know the answers, sometimes wondering why I couldn't think of it yesterday.

I believe the mind even wanders outside the plane that you are normally in, to different levels of consciousness which otherwise in our normal level we cannot reach. This also could be another reason for the brain taking up maximum energy during the sleep. The capacity of the brain to think increases.

Just my 1 cent of thought.