Is it bad for a girl to sleep with a bra?

Not necessarily. There is no reason to think that sleeping with a bra on will lead to harm to your health. However, as with wearing any restrictive clothing, awake or asleep, your growth may change slightly as you sleep. Some people believe that wearing Bras whilst sleeping minimises the size of your breasts. Some people simply find it more comfortable. In the end, it is simply your choice.