At the completion of the downwind, timed leg, you will need to commence a gentle (200fpm) decent as soon as you turn the heading bug onto the runway heading
At night this is potentially dangerous. Unless you are 100% certain of the geographical position of the critical obstruction that dictates the published circling MDA, you should not be descending below the circling MDA until aligned with the landing runway. Commencing any descent below the published circling MDA (slow or otherwise) on downwind or base leg immediately places full responsibility for obstacle clearance upon the captain. It becomes then a case of a "courageous decision" in the manner of Sir Humphrey of Yes Minister.