Wearing a bra while sleeping is neither good nor bad, and the decision to wear one at night largely depends on a woman's personal preference. There is no scientific evidence that suggests that wearing a bra at night is either helpful or harmful to a woman's breasts.
There is no definite link between wearing a bra at night and being diagnosed with breast cancer. Sagging breasts are a result of aging, gravity, breastfeeding and pregnancy. Those women who wish to wear a bra while sleeping are advised to go for brassieres with soft cups and without under-wires. Women with cup size D or above should wear bras for support. However, wearing or not wearing a bra while sleeping does not pose any real risk or benefits to a woman's breasts.