Forget CO2 levels. This is a helpful excuse to rendezvous with your lady/fellow and figure out *some* way to amuse yourselves for an hour in the dark. "Hey, it's for the good of the planet. Or whatever."
Taking a short break every 30-45 minutes can help productivity in general (e.g. http://en.wikipedia.org/wiki/Pomodoro_Technique). It's good to come up for air, check your To-Do list, etc. Keep a small exercise device (dumbbell, exercise cord, whatever) in your office and do a quick set of something whenever your take a break. If you're in a cube farm or other semi-public workspace, stick to silent, cool-looking exercises (e.g. bicep curls). If you have a door you can close, throw in some squats, crunches, pushups, etc.