Mito, do you still get the brain fog if you don’t ride to work? Or, what if you ride easy and don’t do any intervals? So, if you don’t ride and don’t get brain fog then it’s probably lack of glucose as the others have suggested.
Here is my brain fog story - I cured mine by quitting milk. I was putting half and half in my coffee all day and my the third coffee I’d start feeling foggy instead of sharp from the coffee. For some other reason, I decided to quit dairy for a while and I noticed that I had zero brain fog. Also after quitting dairy my sense of smell returned and my sinuses cleared up. I can still eat cheese and yogurt as they don’t seem to affect me in the same way. I was also tested by an allergist for milk/lactose and had no problems. I have no stomach problems with milk. Conclusion - it could be a food sensitivity.