I wouldn't agree that sleeping in an underwired bra is a good idea. I find that lying on my side causes the underwires at the centre gore to bruise the breast sitting on top of them, because the underlying breast tissue will move under the skin. The bra stays in contact with the skin but rubs against the breast tissue and ribs as you twist around. I also find that the cups don't really hold the breasts on the front of my chest when lying on my back - after all, the bra is designed to be worn standing up! You can't really expect it to do the same job lying down, with gravity pulling the breast tissue in different directions, "support" is not going to work in the same way. The only benefit I have found is that wearing a firm but comfortable soft cup bra will stop the lower breast from falling into my armpit when lying on my side - this made a big difference.

@eternalvoyageur - What I meant by the cups not holding the breasts in place is that when you are lying down, gravity is pulling them in a different direction to when you are standing up, thus the bra is trying to support them from the wrong direction. When you are standing, the breasts have to be supported from below, using the underwires to transfer the weight to the band. When you lie down, it is quite easy for them to fall out of the top of the cups because gravity is pulling them *up* your chest and towards your armpits.
With the underwire poking issue, I don't mean that the bra rubs against your skin when you move (that would mean the band is too loose) but rather the breast tissue moves under the skin, causing it to press on the underwires.

thanks for sharing !  It´s always great to hear it from someone who has the experience.