The pronoun fīhi is singular although night and day were both mentioned. Assigning it mechanically to the nearest noun, day, would obscure the conventional relationship between rest and night.
Grammatical possibilities should be distinguished from intended rhetorical correspondences. The two benefits are named without an explicit repetition of “night” and “day,” and the hearer supplies the links.
Thus lack of lexical specification does not mean failure of semantic determination. The art relies on listeners being able to identify the correct relationships without overt pairing labels.