Complex sentence: He worked primarily on nightshifts, overseeing the dormitories and "preventing any flow of traffic after lights out", but found it an "unsympathetic and frustrating job" and left.
Split into simpler sentences: He worked primarily on nightshifts, overseeing the dormitories and "preventing any flow of traffic after lights out". However he found it an "unsympathetic and frustrating job" and left.