Sleeping can cause much hair damage if you are not careful.  Here are some suggestions for preventing damage:

1.  Use a satin pillowcase to minimize friction.
2.  Wear a satin sleep cap.
3.  Loosely braid your hair.
4.  If your braid is longer than your shoulders, try putting it in a soft sock or satin pouch.
5.  Nighttime is a great time to condition your hair - try oiling the ends just before bedtime.

Just wanted to add that Curlgirl64 has her own design for protecting your hair while sleeping.  Her Satin Sox is a satin pouch that your braid or ponytail can be put into which is a great way to pamper your hair at night.