Actually, your brain will work a lot better with ketones, the trouble is with eating breakfast in the morning (we all eat breakfast when we eat regardless of the time break-fast breaking fast get it) if you eat in the morning your cotisol is spiked so straight after your breakfast you get hungry again hence why most people then eat lunch but if you fast till 6 hours after you have woken up maybe have a black coffee or fruit in to stave off big meal you will run better plus take advantage of the 2000% increase in HGH at night and through your fast which preserves lean muscle and sheds fat as you are controlling your insulin and allowing it to use more of you fat stores.
Think about it our hunter-gather ancestors and hunter gathers today do not know for certain when there next meal will come the body knows this so keeps you on the ball during your fast with ketones so you can hunt better,if we did not we would never have left africa, and you never see and overweight tribesman.