Important question: why does it stop at 8pm?
Some nice ideas being tossed out in answer to the original poster's question, but they don't explain why the brain fog stops each evening at about 8pm.
I have the same thing, (my brain fog also goes away each evening) and have been researching the following hormones and body chemicals to try to figure out why:
Cortisol: adrenal fatigue symptoms include low cortisol all day and then a sudden rise in cortisol in the evening)
Serotonin: contrary to common belief, serotonin does not make a person sleepy, it causes a calm, happy feeling -- and more importantly, in those who are low serotonin, a sudden rise in serotonin in the brain can bring about a little energy...hence perhaps lessening of brain fog. Doubt it? Read some of the reviews for 5htp on Amazon.com, a few people there say that 5htp gives them energy, and 5htp is a direct precurser to serotonin.
Dopamine: I haven't done much reading on it yet, but it is a body chemical that increases motivation.
??? Anyone have ideas??
Also, maybe people with candida need the whole day to metabolize the toxins candida spits out, and by evening the liver has detoxed those toxins to the point of lessening brain fog?