There's nothing wrong with wearing a bra while you sleep if that's what you're comfortable with. Some doctors may even recommend it when a girl's breasts are very sore.
You've probably heard all sorts of rumors, both positive and negative, about wearing a bra at night. But the truth is, there's no scientific basis to any of them. Sleeping in a bra will not make a girl's breasts perkier or prevent them from getting saggy. And it will not cause a girl to develop cancer or stunt her breast growth. (It's also not true that underwire bras cause breast cancer.)
Some women want to wear a bra to bed because they think it feels more comfortable. Your best bet is to choose a lightweight, non-underwire bra. 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: that could disrupt your sleep or cause breast irritation.
On the subject of staying comfortable, it's never a good idea (day or night) to wear bras that are extremely tight or don't fit properly. Visit a department store or specialty bra shop for regular fittings while you're growing and developing.