I have 2 different desktops, one for day and one for night (although i freely switch between them if I need to).
They can also automatically switch depending on the ambient light (monitored by the built-in light sensor in the MBP).
These are shrunk down from 1920x1200
I use an application called Nocturne (by the makers of Quicksilver) that in night mode, will remove the wallpaper, convert to grayscale and invert the colors. It makes for an interesting experience. Night mode is great if you have lots of text to read. It's way easier on the eyes.