There was a week in early February this year, right after surgery, where I felt like a million bucks. Normally, I have a pretty low-grade chronic headache (yeah, it's as horrible as it sounds). However, that week, my head felt "clear". I don't know how else to explain it other than I felt like my brain had been washed. I was going to bed at 9-9:30pm, and waking up at 5am, and felt incredible. I reverted not long afterwards, and believe I recently figured out the root of the issue (fingers crossed!), but I 100% believe that you need deep sleep to function well. IMO, it's not sufficient to get light and REM sleep; deep sleep is when true brain recovery occurs. I feel like I should know; before I started treatment, I would go to bed at 9-9:30pm and sleep until 8 and still feel exhausted. You need quality sleep in addition to quantity.
Wrist-based sleep trackers are basically useless, FWIW. I've been using one pretty consistently since before my health issues started and they really can't discern the difference between light/REM/deep sleep phases. If there's anything which mirrors how I feel after waking up in the morning, it's HR dip, but even that isn't 100% reliable.