For ages now I've had one lamp on my desk, to the left of my monitors. I also have a floor standing lamp on the other side of the room behind me. But just recently it started bothering me that the desk light was unbalanced, so I've added another desk lamp to the right of my monitors. It's amazing the difference it's made to how I feel about working there. Everything is evenly lit now and I feel more focused and motivated when I'm sitting there. I think having light shining in from one side was affecting me negatively. Not in a huge way, but it's interesting how much better I feel about my work space now that it's properly illuminated. Weird.