Does sleeping without bra helps in sagging breasts?

Not likely. The benfit of using bras to support your breasts is most likely going to be during the day when you are standing up and the weight of your breast pull on the ligaments that support your breasts. Using them at night will not do much, but you may find it more comfortable to use a bra if you have large sagging breasts.
Awesome question. I am not sure that there is an answer to this question. Intuitively, a person with large breasts would seem to sag more without the use of long term support but I am not sure that this has ever been studied because it would require a long term study with a bra on one breast and not on the other which is conceptually a hard study to perform. I do recommend that women with large breasts wear bras.