The question: Is it bad to sleep wearing a bra? The answer: You've probably heard the unfortunate rumors that wearing a bra to sleep causes breast cancer, or that it causes breast sagging because it supposedly weakens breast tissue. But aside from the fact that it may not be all that comfortable, there's no evidence to suggest that there are negative health effects to wearing a bra to sleep. As Amber Guth, M.D., an associate professor of surgery and director of the Breast Cancer Surgery Multidisciplinary Fellowship at NYU Langone Medical Center, previously told HuffPost Style, “there is certainly no evidence that sleeping in bras is either helpful or harmful.” The cancer rumor in particular began with the 1995 book Dressed to Kill, which included claims that daily wear of tight-fitting bras increased risk of breast cancer by constricting the lymphatic system, thereby trapping "toxins" in the body, Shape magazine reported.