Whether it is by day or night, you still have to descend below the MDA at some point. Only difference is by day you can descend down to minimum obstacle clearance ALT whenever you like, by night you must wait until a continuous descent to the landing threshold using normal rates of descent and flight manoeuvres for the aircraft type can be made.
Either way, the required visual reference remains the same. And in single pilot IFR charter, it is the pilot in command that makes the determination as to whether the required visual reference is available. (Subjective??)
