how to do night time level

Besides for putting point lights on objects you want illuminated, and getting rid of any directional lights you may have, another important feature are sky-boxes. Your unity program should have come with 1 or more, hopefully one of them is of nighttime.

And sorry to bounce off of Gib's answer, I'm not trying to get marked as the right answer, just adding on to what he said

Good luck!

Again, to add to this answer, if you want pure darkness in the game you must go to Edit->Render Settings and set the "Ambient Light" color to black.

I would suggest baking lightmaps with self illuminated textures with a increased value on the Emission (lightmapper) setting. It also helps to have a pale blue directional light with minimum or no shadows (moon light). Point lights are helpful but unless baked can be very expensive. I would also suggest warm point lights for incandescent lighting, and cool blue/green point lights for fluorescent lighting.