I felt like I needed to somehow get some support when I was pregnant even while sleeping, so I did some research. I found the Medela Sleep Bra (LINK).
(Normally I would put a photo here, but the smiling chick in the link freaked me out. Just click the link above.)
They run in the neighborhood of $15 to $20, depending on the size you want, the color, and so on. Amazon didn't have them at the time I was pregnant, and I had only found them on the Gap's website in the nursing/maternity section.
As far as comfort goes - I'll be honest, I still wear these to bed even though I'm neither pregnant nor nursing. They're very soft, stretchy. They're not constricting like a sports bra, it's just a lot like wearing a really snug-fitting t-shirt. On the other hand, it's not as supportive as a sports bra - but you're not running in it or anything, it's just making sure you don't hit yourself or your loved one(s) in the face with your boobs while you sleep.
As far as nursing in this goes, it's really easy - it has a cross-body opening in the front, so you can just nurse really quickly and cover back up at 3 a.m. I'm sure there are other nighttime nursing bras out there, but these were relatively cheap, worked well, and were from a brand I knew. So, since it isn't broken, we haven't fixed it.
After Pickle was born, though, I needed to find a solution that worked well at home, and then again when I went back to work. A girlfriend had recommended these nursing tanks (LINK), found at Target:
Although, to be honest, I'm not 100% sure that this is the right one, since I can't see the photos. Depending on your preference, I would go into the store. I say this because one of the nursing tanks was shaped like this (the orange part is the under-layer when you unsnap and then fold down the outside part of the tank you see above):
As you can sort of see (this is a terrible drawing), the bottom layer is just cut a lot lower so that you can pull your breast out.
On the other hand, one of the tanks has the under-layer (again in orange) shaped like this:
Yeah, that would be a hole just a little bigger than your nipple that you're apparently supposed to thread your boob through. Which, if you're nursing, is stupid and limits skin-to-skin. And, if you're pumping, you're supposed to somehow fit the flange through there, or get enough of your breast out to get into the flange. Just - no. So be careful of what your preference is. I couldn't take these seriously enough to use them when I accidentally ordered the wrong one, so I took it back.
I used the nursing tanks a ton, however - even after I went back to work, I continued to wear them (the top one, obviously) for a considerable amount of time. I felt like I was more covered in my stomach area when I pumped, just on the off chance someone happened to walk into the room (which did happen once). They're about $20 or so - I believe there are similar options elsewhere, we just had a Target local.
I officially switched to a supportive bra later, however, as nursing tanks sort of leave you flopping around. I did avoid an underwire (even though I need one for support) because it can affect production.