I have experienced the same problem, and only recently have come to what seems to be a solution for me. I notice in a lot of movies and tv shows where it is a night setting, that actually there is quite a bit of light and that darkness is suggested - often through the color of lights use, namely blue, to light the scene. Also, shadows tend to be harsher and the area of focus more brightly lit than the areas to be more obscured due to the effects of darkness. Also, blurring the details in the background seems to help. I might add to the above, I may also add more grey to material colors of the textures to make them more dull and subdued as would be expected in low light settings. Studying old b&W film noir movies has helped as well since most of these were shot to suggest darkness as the main atmosphere. Hope that helps some. :O)