No, the casserole doesn't need to sit overnight. However, letting it sit overnight will give you some really wonderful results. u003cbr/u003eu003cbr/u003eMany recipe recommend resting the casserole overnight so that the bread has a chance to soak up all of the egg mixture. To speed up the process, simply press the bread down with your hands like a sponge. When you release the bread, it will begin to soak up the egg mixture. Use clean hands to gently separate pieces of bread to make sure that each piece is coated with egg mixture.