I could never find anything that i was comfortable sleeping in - until i found these things:

for leaking breasts - BellyBelly Forums ~ Pregnancy, Birth & Baby

and a few other members recommend them here: Breastfeeding tops /tanks..? - BellyBelly Forums ~ Pregnancy, Birth & Baby

(it should highlight them in red for you, there are quite a few posts in the thread)

they are singlet tops, with a special lining, which absorbs the milk so the outside doesn't get wet at all... So comfy (i wore it under my pjs), and it has a zip in the front so you can b/f!

A lot better than wearing a bra to bed... i absolutely love mine, one of the best things i bought TBH.. only regret was i got them at like 2 month, and i had those first 2 months of not sleeping very well!

Hope that helps.