"Don't sleep with your hair up. The hair is not able to move as it is held tight in an elastic. When your head is moving around throughout the night it may break hairs.
Don't put wet hair up, either. Hair is stretched up to 30 percent longer when wet. When it dries and shrinks back to normal length, an elastic could easily break."—Jennifer MacDougall, celebrity hairstylist/L'Oreal Professionnel