One of the things I've learned from measuring my brain with my Circl headset, is that I am very good at adapting to different cognitive states. This is both a good and terrible thing.
It's good because humans are able to adapt to their surroundings and changing environments to continually thrive as a species. We've survived an ice age because of this.
It's bad, because when we get tired, chronically tired, we redefine that as our normal and feel, well, normal. We don't realize that we are operating with an exhausted brain.
I've measured this with myself and I can see my brain function is half of what it was last month, but I feel roughly the same. This for me is a very compelling reason to measure my brain and make sure my normal doesn't continue to slip.
In fact, I've had to stop drinking coffee which I love so so much. For me, I get burnt out because coffee helps mask how tired I am. I relapse, of course, but about a week or two later I'm useless. So, for the past two years I have (mostly) abstained from coffee.
I hope you have found a healthy and balanced normal that allows you to bring your full mental faculties to bear on living and enjoying this life.