The world is a setting that pulls a Jeckel and Hide from Day to night.  Though the day is far from safe it is a calmer time and the ability to see the colors and life that has been birthed back into the cement and metal world of man.  By contrast the night is filled with creatures specifically bent on hunting down and killing every last surviving human as violently as possible.  The constant feel should be a dark gloom of hopelessness lightened only by memories of a dead world.