I used to think darkness, but it turned out that quiet was the active ingredient in darkness.

Quiet is the most important but even when I can't get that it still helps me to be in a dark and cold environment. It's calming.

It's nice to be able to look away from a computer screen to think. So facing monitors and a window at the same time is ideal. I find this setup good for day or night, and have a similar setup at home. I took a quick picture to show what I mean. http://flickr.com/photos/ikirigin/1061406827/

I constantly have headphones on. Sometimes without music -- lowering interruptions.

In the light, but when it's dark outside. When it's dark inside, the brightness from the monitor gives me a headache.

Light, but only natural sunlight. Darkness is better than ugly green fluorescents/ugly yellow incandenscents.