How would you make light?

I have been thinking about it and I am completly stuck. So how would you do this?

Your question contains insufficient information to ensure an accurate response. If you can describe what light effect you're trying to accomplish then maybe we can narrow down the options.

Sorry for not giving enough information. I wanted to try and create night and day. So you know when its like 6pm it gets dark. But if you place a torch it will create light so you can see better. I have already done the time.

The simplest solution I can think of is to create a "darkness" actor the size of the screen and anchor it to the screen, then adjust its opacity/alpha as necessary.

You won't be able to make "pockets" of light this way, but if you check the player's distance from all possible light sources, you can brighten the entire screen when the player is close to a light source.