During the beginning phases of your fast, your mind might be a little bit cloudy. However as you progress and your body enters ketosis (usually around the 3rd -4th day) many people report having enhanced mental clarity.
If you are coming from a carb-fueled diet, you might get some fog. Keeping up on electrolytes will ease that somewhat. The fat-fueled brain is pretty much clear and focused always- at least that’s been my experience over the former.