As per August 21st, 2007, we have decided to change the default setting for ambient lighting on night maps. The new settings are:
"light" "35"
"ambient" "0 0 0"
The effect of this is that night maps will get much darker. There will still be some ambient lighting, but it will be pretty dark. Shadows and indoor areas will be pitch-black. This will add more atmosphere to night maps.
Something to note here is that when you use model entities on your map, those models should always be in an area with nonzero lighting. This is because even in pitch-black areas, models are still slightly lit, which doesn't look good.
If the model is placed outside, it is enough to ensure it is not in the shadow of anything else. If it is, or if it is inside, make sure there are light entities nearby to illuminate the model.
UPDATE:
With the new map format, these settings are built-in and no longer have to be manually set.