According to Kids Health, doctors advise their patients to sleep in a bra if their breasts are sore. Whether or not an individual wears a bra during sleep depends on her specific comfort level. Sleeping without a bra does not cause the breasts to sag.
The Maurer Foundation explains that there are several myths that link breast cancer to women who sleep with their bras on or wear their bras for extended periods of time, but there is no scientific evidence to back up these claims. These breast-cancer myths stem from an unproven belief that bras compress the chest, which in turn can block the lymph nodes and create a build up of toxins in the body.