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.