#1 figure out your city shape and size.
#2 figure out the lighting level of area... For example, a peasant is only going to use a fire lit around their hut maybe while the center of the city might have giant bonfire that is lit every night. Modern Cities are generally going to have lights along their roads with downtown areas being lit up by bilboards. The more active night areas are going to have more light and thus the overall brightness is going to be higher than as you travel out ward. Roads themselves depending on the era may be lit or may not be lit.
#3 Every 30 to 50 kilometers there will be at least a little light along the road because there should be small town of some sort there. Major cities are going to obviously be very bright as there is more light while hamlets, villages, suburbia will be less bright in the overall depending on the era.

Now... to make it look natural what you need is the understanding that light is radial usually. It's brightness and size is largely determined by distance. It follows the ... i think its called the square law. Basically that means, let's say you have a light that fills an area of 10x10 cells with 100 lumens (light units) at 10 feet... So each cell has 1 Lumen at 10 feet. Now if you step back to 20 feet the grid exapnds to 20x20 but still has 100 lumens in it which means that each cell has .25 lumens in them. In other words, the brightness is divided between all the cells and there are 4 times the number of cell because you doubled the distance resulting in a quarter of the brightness.

Now let's say there are 2 light sources 30 feet a way from each other and you are in the middle of them. Step back so that you are 10 feet from both and you have same result for both above... step back to 40 feet and what happens is that you are now at a point with the 2 light sources overlap. This results in 75x40 grid. Most of the cells will have .125 in them, but in the center 5 columns you have cells with .25 lumen in them. So you have a brighter spot than what it looks like there should be.

So how does this help? Well consider that all this leads us to produce the question... How many lumens are in the area. That depends on how many light grids in the area are overlapping each other. With each new light overlapping a given area the particular area should get brighter. This can be accomplished multiple ways I'd think. 1 way might be to have a brush that goes on lightly, but every time you go over it increases how much gets laid down. Another way is to create layers that have transparency and then have different layers for outer city moving towards the downtown inner city... Another way that I think would be much cooler but probably harder and more tedious is to lay out lights in the city where they would be with different layers representing different types of lights and then us a layer style to give an outer glow that you can adjust allowing you to adjust the height of the map without having to redo where all the lights are.

Hope that lecture on how light spreads helps ^.^