By day provided you have the required vis and 300 feet obstacle clearance you can circle as required to intercept the runway centreline for a landing. So in this case you would descend to the runway approach minima and circle with the 300 feet obstacle clearance, by night you'd descend to circling minima if you needed the reciprocal runway or take the runway approach and land with a tailwind. Operationally you would have a rough idea of the cloud base and if conducting the approach at night and planned to circle on a runway approach you would descend to the circling minima and if not visual and the downwind was acceptable, continue to the runway approach MDA and land if you are visual.
I don't recommend circling at 300 feet unless you're at an aerodrome you know well or in something very docile! (C172)