You shampoo, condition and treat your hair with TLC, so nightly protection should be on your list of things to do. It's not enough to only care for your hair during the day. You have to protect it once you go to bed as well.
Sleeping on regular cotton pillowcases will rob your hair of moisture. Black hair rubs against cotton and the rough cotton fibers (no matter how soft they feel to your hands) can catch and pull your tresses, leading to breakage. You can still sleep on those cotton pillowcases, but put a layer of silk or satin between them and your hair.
Satin or silk caps and scarves will go a long way toward keeping your hair in great shape. These materials are gentle to your hair, gliding against it instead of tugging on it. If you're a "wild" sleeper and have trouble keeping coverings on your head, hairpins can help hold them in place. You can also opt for satin or silk pillowcases if you don't want to wear a hair cover (especially if you have a partner who complains that this look isn't sexy!).
It only takes a minute to slip into a soft hair cover, but it does so much in helping you wake up with softer, well-moisturized hair that will see less breakage and dryness.