One of my clients would tell me about his "Brain Fog" and attribute this condition to his problems with writing. He would describe his tired (and somewhat depressed) mood in great detail, and would catastrophize being a bit tired, unfocused and "down" by actually viewing it as a condition over which he had little choice and control. After a good deal of work, we were able to help him change that!
To help him start to view his "condition" in a new light, I referred to it as "San Francisco of the brain"--he found it funny, and this helped him be non-defensive as he started to realize that perhaps his "condition" was not something that he had no control over. This allowed us to craft a plan full of strategies for moving toward consistent morning writing: Self talk to help reduce his awfulizing and challenge his expectations of what "Must" occur (I must be sharp and totally "on" to write), a few minute of exercise to help him energize, and rituals and habits to help him stay in process.
Overtime, he began to see "Brain fog" as something that he could write through, and that he had choices.