Mental clarity dropping before burnout?

Something I've been picking up on lately is that burnout isn't all about feeling super tired first off. For me, it actually starts with my brain getting a bit sluggish-thinking slower, second-guessing myself more, and having trouble focusing on even the simplest stuff.
Physically, I might still be good to go, but mentally everything feels like it's dragging me down.
It makes me wonder if these mental shfts are like an early warning sign before the fatigue hits. Anyone else notice their brain getting fuzzy before they feel totally burned out?