I really like nighttime shots, but I think that instead of just making the whole picture dark, you should practice with some different lighting techniques to add a bit more character to the scene. Right now, everything is just sort of universally dark, which makes the pictures seem a bit flat.
I don't have much experience with it, personally, but a few fellow Garhims have done special lighting to great effect. These are some good examples:
"Planning is Needed!" by Brother Steven
"Sir Angus Finds the Sword" by Brother Steven
(This one isn't quite nighttime, but you can see a good technique in use at least)
"Library" by Atahlus
There is also a good series of pictures by Invicta Bricks (Thomas Arrowford) that is from their "Operation Russel" where he uses either some sort of post-edit filter or maybe a blue light to accomplish a very similar effect to what you're doing. You might get in touch with him to ask how he did it.
"Operation Russel" by Invicta Bricks
Keep up the good work!