text: why aren't you supposed to sleep with a bra on? i don't, but i always hear everyone saying this and i don't know why most people think it's just uncomfortable....there was a rumor it caused breast cancer...it was never proved...but also for some reason it makes your boobs grow smaller...my friend learned that the hard way and she started sleeping with her bra off and her boobs got bigger...so just overall it's probably better to sleep with it off...