The answer is: it depends. The first two weeks while the body adapts, can have worse things than brain fog, and they are known as the keto flu. Some people ca

The answer is: it depends. The first two weeks while the body adapts, can have worse things than brain fog, and they are known as the keto flu. Some people can suffer from it, others are just fine all the time. After your body adjusts, you can feel sharper for a longer time than in carb heavy diets. In my particular case: I used to work fine for about two or even three hours after lunch (getting my carbs to be able to work). Now, with a keto diet, I can work for eight hours straight. I am and feel sharper. I have been in Keto for years now.