Halakhah for Yevamot 148:7

From the laws of the commandment is that which they, may their memory be blessed, (Mishnah Mikvaot 1:8) said that the waters of the mikveh are fitting to purify in them from any impurity, such as [that of] the menstruant, the zavah and the other types of impurity of men and vessels - except for only a zav, since Scripture made explicit only for it [that] living waters are his purification - the understanding of which is gushing water. And also that which they, may their memory be blessed, said (Yevamot 74b) that even though the immersion of some of them is during the day, their purification is not completed until the sunset, as it is written (Leviticus 11:32), "it shall come in water and be impure, until the night and it shall become pure." And he needs to immerse his body revealed - meaning to say, that all of it is [in contact] with the water, meaning that there not be anything separating between all of the body and the water. But if he immersed in his clothes, the immersion counted for him, ex post facto - since the water goes into them. And the menstruant is also [included] in this law, ex post facto, and is permitted to her husband - and that is so long as the clothes are not extremely tight. And the immersion of all of those obligated to immerse is during the day, except for the menstruant and the woman that has given birth, such that their immersion is during the night. And one impure from a seminal emission may immerse from the beginning of the night and on, until the sunset [of the next day].