Hi, i would say this is because the action words (rest / recuperate) are doing the action being talked about. I guess you could say - most peope incorrectly think the brain rests or recuperates while we sleep. That would also be correct but it doesnt flow as well as the first sentence.

We use "Present Progressive or Continuous" to indicate an action that continues. During the time a person is sleeping (8 hours), the brain is not resting. During the continuous process of sleep, the brain "is" busy. Think of the word "recuperate". It is a time "period" of recovery. Again, this is a continuous action. Hope this helps

I think the person writing the sentence wanted to emphasize the "state" the brain is in when sleeping. Knowing the sentence after this one would give us more context to guess why it was worded this way.

I think you could correctly use both. The simple present is used for habitual or recurring events. The present continuous is for ongoing events in the present.