One of the (many) things I envy about girls with breasts like tiny mandarins, is that they don't have to worry about nightwear. They can wear whatever they want and if they have to answer the door to the postman or the milkman or the Milk Tray man (where IS he these days?) they don't have to do it with their arms folded because their breasts are unsupported.
Naturally, big-breasted girls can do this too, but personally, I don't like wondering round the house in my pyjamas with unfettered breasts.
Also if your breasts are really large and/or you're breastfeeding, it really isn't that comfortable sleeping with no support at all. Many women really don't like wearing a bra at night but you don't want nuffin, neither. I know this from years of co-running a parenting board (no not that one, this one).
Now, you can get pyjamas/nighties with 'secret support' - Bravissimo being the most obvious - but they are all sleeveless, and I like to have my shoulders covered at night. But also it really limits you to the styles available (which you may or may not like) at a time when, as a big-bosomed woman, you already feel you have limited choice in clothes.
M&S now does (and has done for a while) some secret support camisoles that are really rather good and these work really well at providing a modicum of support and you can wear them on their own, atop a PJ style trouser, if - unlike me - you don't mind having your shoulders bare. Or under any normal PJ you like, thus opening you up to choose from any PJs out there. You could I guess also wear them with nighties, but I don't really do nighties.
My favourite M&S secret support camisole vesty things are these and these. Both are £12 a piece and come in a variety of colours, the shoulder straps also adjust, a nice touch.
Although Bravissimo does a racer back PJ top in lots of larger sizes, so it fits better than the M&S ones if you're big-cupped, they are over double the price and always sold out. It also has great strappy tops, but these cost even more: £32. So I think the M&S ones compare really favourably, especially if all you want is something to wear for a bit of support under your current nightwear without going to bed fully upholstered in a bra.
If you've found a solution that works for you, do share..