Recently I had to create a night time shot out of one taken at daytime!
So heres how you do it
First off take your original image.
Duplicate the background layer
now to get the night-time effect that we want
We now go to Image>Adjustments>Variations and this window should appear
Click on More Blue Twice and Darker 3 times (this are my preferences, you may want to change it a little depending on your needs) and this is what you get
If its not dark enough however add a Levels adjustment layer above the Night Layer
And work the Middle Slider until you get it as dark as you want
Its simple and basic but powerful. There are better and more interesting tricks to turn day into night, but this is a good one for ones that don’t have any light source in the image.