I used to have a lot of brain fog particularly in the mornings & it turned out the cause was anxiety. I wasn't sleeping full nights either so that didn't help either I'm sure.

I didn't even really know I was suffering anxiety problems at that time, it wasn't until it got up until the point I was diagnosed with GAD that I knew. Strange how one can be oblivious of mental health problems sometimes.

I've since been on an SSRI, come off it & now no longer have issues with brain fog. That's what makes me so sure anxiety was the cause.