Maybe you're just getting your sleep stages confused, but REM sleep, while considered important, is not the vital function of sleep related to memory, dementia,

Maybe you're just getting your sleep stages confused, but REM sleep, while considered important, is not the vital function of sleep related to memory, dementia, and other health outcomes - from everything I understand, and I work in the sleep/neurotech space. N3, also known as deep sleep is when the glymphatic system flushes toxins from the brain, consolidates memories, increases HGH secretion, along with other hormonal changes, primes the immune system, drives parasympathetic response, etc etc. REM sleep is related to emotional processing, and some memory, but I also recently heard a theory that REM may also be necessary to prevent the elasticity of the brain from over-writing the visual system with other inputs, which was an interesting theory, as sight is the only sense which is turned off during sleep.