Heather, good point - if you got the blahs, make sure you get some sun time... open the blinds, move the shutters, whatever... make sure you can get some sunlight into your rooms and into your workspace. If you work in a closed office or whatever, take a couple of good breaks a day and get some sun on your face... it will recharge you very quickly.
Fresh air is just about as important - if you work at home, make sure your air conditioner and forced air heat system filters are clean and fresh. Make sure there is enough humidity in the air where you work. All things that make your body and brain happier, and create a better productivity...
A little Feng Shui never hurt, either.
Alex