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

The current top theory of why sleep has evolved as a necessity is:
1. In the past (and even now), organisms had periods where remaining active actually provided no survival benefit. Think to when an organism has just fed. Why continue producing energy and remaining active? If anything, it only puts the organism in unneeded danger.
2. Number 1 above led to organisms developing an “off” mode, where energy consumption was severely diminished. Over time, this evolved into what we now know as sleep.
3. The off time that this mode provided, naturally provided a good time to regenerate / perform many maintenance type tasks. These likely evolved simultaneously with sleep to occur at the same time.

Evolution explanations are always “just so” though, we don’t actually know this happened.