Don't sit behind your computer at lunch time.
You've been there all day.
I know it makes you feel more productive as you eat your sandwich and type with one hand (not to mention how hardcore it makes you look to your other work mates) but it's silly. You're not being as productive as you think. Productivity is far more than just about how much you get done, but about the quality of work you're producing, and quality is hard to produce with a tired mind.
So here's what you should do next lunch time.
Get off your computer and get a few of your work mates (preferably ones you like) and commit your lunch time to getting outside and taking in some fresh air. Walk for half and hour, 15 minutes, heck even five minutes. But just let that outside air hit the refresh button on your productivity.
It's amazing how complex problems seem simple after a walk at lunch.