This advice was from an old high school math teacher which worked well in my lifetime. You want to sleep well 2 days before the exam because you probably can't sleep the night before with all the adrenaline. You want all your blood going toward your brain. Eating too much would make your body work on digestion. Do not eat proteins before the exam.
Proteins are harder to digest than carbohydrates.
Do not drink too much coffee before the exam. Besides not wanting to run to the restrooms while taking exam, you don't want to be too wired for the exam. Make sure you have enough pens, pencils, whatever. The last thing you want to do is get distracted by silly little things. For everything we do in life, we need to warm up.