What do scientists mean when they say “we don’t know why we need to sleep”?

Question

I see this a lot but isn’t the basic answer just that the brain needs to recharge and clean out toxins within the body?

In: 1

Answer 1

Why does the brain need to recharge? Why can’t it do that while we’re awake? The body regularly clears out toxins while we’re awake, so that can’t be the full story.

It’s unclear what exactly the brain needs to do when we’re asleep that it can’t do when we’re awake.