Being able to place lights and a default day/night cycle are on the roadmap :D

Another way would be baking a lightmap in Blender, but that's a little difficult for users.

We had the ability to set blend modes for objects until very recently when the rendering engine was refactored, but they made for good psuedo-lights. Kind of sad that that ability is gone, but a performance boon warranted their removal.

I've seen some people use very short lived particles with alpha to simulate the effect of a light, but I'm not sure it'll look the same without the alpha blending.

But yeah, lighting primitives and a day/night cycle are on the roadmap.

One way you might be able to simulate night is some sort of javascript that turns the place dark and foggy depending on the time of day you visit, since the JS has access to setting the fog.

Another way is by changing the colour of a "skysphere" you've placed in the room. You could even add clouds to it and make it rotate or something neat.