Laff itemNashr expressionRelationship
al-layl (night)li-taskunū fīhiRest and repose at night
al-nahār (day)li-tabtaghū min faḍlihSeeking livelihood during daytime
The two purpose clauses convey separate benefits. Rest and the pursuit of livelihood are both human concerns, and their natural relation to night and day gives the figure its ordered structure.
The antecedent of fīhi raises a grammatical question. The rhetorical pairing is not resolved by simply choosing the nearest noun: contextual sense also matters.