There's nothing wrong with wearing a bra while you sleep if that's what you're comfortable with.
Sleeping in a bra will not make a girl's breasts perkier or prevent them from getting saggy. And it will not stop breasts from growing or cause breast cancer.
Some women want to wear a bra to bed because it feels more comfortable for them. Your best bet is to choose a lightweight bra without underwire. Some camisole-style pajama tops even come with a bra built in. The bra you choose to sleep in shouldn't be too tight or have parts that dig in. An uncomfortable bra might make it hard to sleep or irritate your breasts.