
Using a material such as cotton will suck the moisture out of your hair leaving your hair dry. Using cotton pillowcases also causes friction which can lead to excess breakage.
So while it is important to protect your hair by covering it while you sleep, it is also important to use a material that won't damage your hair. And you can achieve these two things, by wearing a satin/silk scarf or bonnet or by using satin/silk pillowcases. Which helps keep the moisture in your hair. It also causes less friction thereby decreasing breakage and therefore helping you retain length.
Covering your hair also keeps your hair neat lessening tangles in the mornings. Making it easier and more manageable saving you time in your morning routine.
