Second vote for electrolytes. I take magnesium depot tabs at mealtime and also take some salt (potassium and natrium mix) during fasting times. Headaches during fasting have stopped completly.
And: do you drink enough?
At the beginning I had this brain fog sometimes, I think it was my body getting used to fasting. I ate later in the evening (half past eight) and slept over the low point. Now it does not happen anymore.
Third vote for electrolytes.
When I start getting the pangs, which I sometimes do when I wake up or within a couple hours of doing so (my eating window ends at 5pm) I do notice a difference if I take some electrolytes (iodized salt and NoSalt) and/or a small (like, teaspoon) of apple cider vinegar. Gets me out of that woozy feeling pretty quickly.