There is no scientific evidence to support the claim that sleeping with a bra causes breast cancer. Breast cancer is a complex disease with multiple risk factors, including age, family history, and certain genetic mutations. Wearing a bra while sleeping is a personal preference and may be uncomfortable for some women, but it does not appear to increase the risk of breast cancer. It is important to practice regular breast self-exams and to receive regular mammograms as recommended by your healthcare provider to help detect breast cancer early.