Wow, you're making some pretty wild connnections here,Tracy. The main reason people who exercise more have less breast cancer is that they're generally thinner and have less circulating estrogen in their system. I've never heard of a study that addressed whether the position a woman sleeps in has any bearing on her risk of breast cancer. - PJH
It is so easy to become overwhelmed by all the things that have been linked to breast cancer, at least potentially. However, as far as I can discover, sleeping on one's stomach is not one of them.
PJ is right about exercise and estrogen. That's why exercise has been proven to help prevent recurrence in women who had estrogen positive breast cancer but not for women (like me) whose cancer is estrogen negative. Exercise is still really valuable for all cancer patients (and everyone) though, for a whole host of reasons both physical and mental.
As for sleeping - I think it is safe to say that the position we sleep in shouldn't keep us up at night!
Take good care-