This is a highly debated question that I explore.
I often come across women who are most concerned about the well-being of their breasts. No woman wants sagging, let alone early in life. No woman wants to do harm to her chest!
Most women are concerned with sagging and exhaust themselves with finding a great bra to keep things in place. It shocks me how often I have, in my lifetime, come across the matter of sleeping in your bra.
Gravity is an enemy, but keep in mind when you are sleeping, your body is horizonal (unless you sleep in some odd way I am uninformed about!). This means, if anything, sleeping on your side and stomach is bad for your breasts, but it really is not an issue.
The best way to sleep is, of course, on your back. Like this, your breasts have no where to sink except into your chest.
There is nothing sleeping in your bra will do except cause a lack of comfort and poor blood flow.