You can assign time limits to each category you make (right click a category and go to "Wallpaper Categories" -> "Edit Category"). So to achieve what you're after, you would be making for example a "day" and a "night" category.
For the time limit of the "day"-category you put something like:
* from: 8.00
* to: 16.00
And for the "night"-category:
* from: 16.00
* to: 8.00
That should work. Let me know how it turns out