That's very interesting... I have just built a night time scene of a tiger creeping through a ruined city. What I did to give it the shadowy effect, was just to paint the whole diorama a slightly darker shade (including the model). My next step yet is to build a shadow box around it and a small filtered light to go on top. I will then put an orange filter over the light to give the scene a hazy, burning, city-orange.
My diorama is very easy though, because it was all done in 1/144 scale!
For your project remember, shade darker and use the least amount of light possible to hide subtle details in darkness and give the scene the eery scense of night.