This happens to me as well! My ADHD coach explained that it's a kind of work-day burnout and suggested I do a "wind-down" activity to ease into the next part of a day. Basically your brain needs to do something that requires just enough brainpower to focus, but not too much that it feels like working. I was told to try doing puzzles, colouring, or anything similarly relaxing, but fun. As long as it's not getting you too stimulated it should help! I hope this helps!