The union of husband and wife in Gan Eden:
The union of matrimony is not only practiced in this world, but is also practiced in the next world, in Gan Eden. Those couples who were married in this world continue their relationship in Gan Eden as souls, and just as their union in this world brought about physical children, so too, in Gan Eden their union brings about spiritual children. The time of marital union [i.e. Zivug] of these souls is every midnight. For example, the souls of Avraham and Sarah unite every midnight and give birth to souls of converts. These souls eventually descend into the bodies of converts and bring them towards a Halachic conversion, to make them into Jews. Now, just like in this world you have couples who cannot have children, so too in the next world there are souls who are barren and cannot produce souls.
 Mamarei Admur Hazakein p. 151 “Lehavin Inyan Zivug Haneshamos”