Which period constitutes the primary basis for the division of nights among wives?

Primary text

The primary basis for the division (Qasm) of nights between wives is the night itself. This is established because the night is conventionally reserved for dwelling, seclusion, repose with one's family, and conjugal relations. The Day is for livelihood, movement, earning, and engagement in affairs. Evidence for this is found in the Quran: "And of His mercy, He has made for you the night and the day that you may rest therein and [by day] seek of His bounty" (Quran 28:73), alongside "He it is Who has made the night for you that you may rest therein and the day, sight-giving" (Quran 10:67) and "And made the night a covering" (Quran 78:10). Therefore, a man divides his time by assigning one night to each wife, engaging in his daytime activities, fulfilling public obligations, and other permissible pursuits. However, if his livelihood requires him to be active at night, such as guards, the day becomes the basis for division in his case, just as the night is for others.