La-ʿamruka is an oath formula in early Arabic poetry, literally invoking the addressee's life. In mā amrī ʿalayya bi-ghummah, mā negates the proposition; the bāʾ before ghummah reinforces the negative sense.
In nahārī wa-lā laylī ʿalayya bi-sarmad, the pairing of day and night heightens the declaration: his affairs are not permanently veiled and he does not regard the night as endless. The force of the construction lies in its emphatic denial.