Wearing a bra while sleeping will not make a girl's breasts perkier or keep them from sagging. It also won't make a female acquire cancer or hinder her breast development. (Underwire bras do not cause breast cancer, either.) Some ladies prefer to sleep with a bra because they believe it is more comfortable.
Your sleeping posture may alter your bosom when braless, but it will not enhance its size. The breast ligaments stretch over if you sleep on your stomach, side, or back for a long time, creating sagginess.