I wear my underware and Bra all the time. I can think of only 3 reasons it should be off:
1 Shower or bath
2 Giving Birth
3 Doing what caused you to give birth in the first place!
My mom is also one of those who believes that things need air but I just can not sleep that way. I usually sleep in a t shirt and my undies and for the record I usually sleep diagonally across the bed facing the bathroom. I work 3rd shift so I get the bed to myself.