It's a well-known fact that we derive energy from the food that we eat. Like all the other organs, our brain too, processes the food that we consume. So, if you are somebody who is preparing for an examination or have a child who is preparing, then you should try foods that might be very beneficial to your brain.
Include protein-rich food with complex carbohydrates in your diet. These will greatly increase your mental alertness. On the day of your exam and the days preceding it, eat a lot of eggs, nuts, yogurt, cottage cheese, etc. You can combine these with foods rich in complex carbs like brown bread, roti, fruits, oats to get constant energy with better concentration.
A hearty breakfast is a must before your exam. A good combination is whole-grain cereal with low-fat milk, eggs and toast with jam, porridge and boiled eggs, oatmeal and fruit yogurt, or sugar-free muesli with scrambled eggs.