[outworlder]

Yeah. For starters, there is a whole bunch of toxins that build up when you don't sleep enough. But it can't be the whole story, as why wouldn't we have mechanisms to remove those and keep the brain functioning?

It can't be all about memory retention. There are some fascinating studies on how some groups of neurons repeat the same patterns they did when the organism was awake, presumedly for long-term storage. But why can't storage happen while awake?

There must be a reason why these "brain batch processes" run while most of the brain is shutdown.

[brazzledazzle]

I've found that it's best to not think that evolution optimizes for the best thing but rather that it optimizes for "good enough". Sometimes they are one and the same but they don't have to be.

Sleep could have started as a rudimentary adaptation to one thing and ended up being refined and coopted for other things.

[Freestyler_3]

Evolution is diverse, if all living things need sleep then I think its A: something that is absolutely needed, without it you wont survive [guaranteed] B: A left over from our common ancestor

[Noseshine]

    > why wouldn't we have mechanisms to remove those and keep the brain functioning?

I don't know if it's in the links below, but I remember hearing a talk that the flushing required physical changes inside the brain: Widening of channels (by neurons getting smaller and making room, if I remember that correctly?). So normal function can't go on while the flushing process is being performed.

https://www.nih.gov/news-events/news-releases/brain-may-flus...

https://www.nih.gov/news-events/nih-research-matters/how-sle...

Since we have distinct sleep phases we may not have just one "sleep", (very) different things going on at different phases.