Making images taken under daylight conditions appear as if they were taken at night using image processing elementary techniques
That really doesn't make sense: you'd be training a neural network to mimic a conversion algorith, which you already have. That's a waste of electricity ;) I doubt the resulting nets would generalize at all: you're not even training them with actual day/night picture pairs. So, really, as usual: one of the most significant problems in machine learning is getting a good set of input data. 

In your case, that's still pretty easy (just take a couple hundred pictures from the same positions at day and night – you can basically do that in a week, if you have a camera that you can reliably bring to the same positions). So, don't skip on that crucial step.