When you sleep on a cotton pillowcase, the tossing and turning causes friction between your head and pillow. This can result in hair breakage. Using a silk/satin scarf or satin pillowcase will greatly reduce that friction and maintain moisture in your hair.
As an added bonus, stain pillowcases and sheets help to maintain the moisture in your skin, too! Making the switch from cotton to satin will protect both your curls and your glowing skin from moisture loss and friction.