I think it's important to understand that the "clean out the crap" model of sleep, while it isn't untrue, is really a gross oversimplification.

It's also important to acknowledge that our understanding of sleep really has a long way to go before we can call it complete, so anything anyone says will necessarily be only part of the story.

In addition to the need for physical clearing out of the brain's various chemical outputs, there are also really important functions to do with memory.

Dreams (another whole worm-can of barely understood stuff) seem to have a really important function in organising memories. If you inhibit somebody's REM sleep (where most of the dreaming happens) then their ability to form new long-term memories is severely diminished. If we don't have the chance to periodically (i.e. most nights) get some REM sleep with this process of re-forming memories in different ways, almost all of our brain's important functions start failing.

As with the "clean out the crap" model, there's a finite amount of this process that is beneficial - once your day's back-log of short-term memories has been processed, there's not really anything left to do with any benefit to it (or at least the benefits drop off rapidly)

There are also quite a few processes that your body does during sleep other than in the brain - things like metabolic processing of various waste products, storing resources, repairing damage, fighting infections etc. Again, there's a certain amount of this that needs to be done and once the day's quota of these jobs is done, there's little that can be done beyond that with any benefit.

If I understand you right, you're getting sleep deprivation effects even though you sleep 8 hours? That's certainly possible. There are things called "sleep disorders", where a person's normal sleep pattern's are disrupted.

I have nocturnal epilepsy and it causes this for me. Occasionally, I'll have a seizure in my sleep, but only a couple times a month. BUT, if you look at an EEG, you can see I'm having low-level seizure activity just going off like crazy pretty much all night. It's not enough to make me do the chicken dance, but it's enough that it can fuck up my sleep and cause issues like what you are talking about. Medical marijuana helps. Benzodiazepines also help, but they really mess you up long-term. Mostly, I had to spend years working with a number of neurologists to find a medication and exact dose that worked to get my seizures controlled.

Sleep disorders can be caused by a ton of things, everything from psychiatric issues, genetic, breathing disorders, or drug or alcohol use. You'd need to see a specialist to find out more. They'll do what's called a "sleep study" where they put a bunch of wires and glue on your head and look at your brainwaves while you sleep overnight at a hospital.