Inverting colors using Control-Option-Command-8 made it just slightly more comfortable, but I didn't fancy the weird colors. Calibrating the screen to a lower gamma didn't do much either. In the end, I found these two freeware utilities: Both act by applying a color filter to make the screen look darker. I like Nocturne because it activates/deactivates darkening with a single click. I assigned a trigger to Nocturne in Quicksilver, and now I can get a dimmed screen with just a keypress ... and no more bloodshot eyes!