Green tea is a great option as it has far more caffeine than regular coffee, but leaves you calm as well. Avoid fast-digesting sugars, as they will cause your sugar levels to peak and dip, leaving you tired and unable to concentrate. Make sure you have regular meals of healthy carbs and fats and lean protein. Snack on veggie sticks and hummus, whole fruits, raw nuts (almonds and cashews), and welcome breaks in concentration every 45 minutes.