Even during night time, there is always a slight white source of light. The moon provides the most obvious illumination, depending of course on which light. And the eyes are tenfold more sensitive to light as they are during the day. While details still need to be indistinguishable during the night (such as survivor vs zombie recognition), the landscape needs to be visible at the slightest and a the sky clearly distinct from the horizon in order to achieve the least bit of orientation. People are right to complain about this and anyone who is implying that it is not a problem simply forgot that he turned his gamma to maximum a while back to actually allow him the play during the night.