It's even worse than lowered productivity for me. I just start getting straight-up depressed; just absolutely terrible feeling. No energy, no drive, no light. And then I realize that somewhere in the last two weeks I stopped my regular exercise regiment for some reason or another and pick it back up - cured instantly.
It's easy to fool yourself into thinking that the funk caused the stop in exercise but that's nearly never the case.
EVERYBODY, hacker or not, should be getting regular exercise. Please. You won't regret it.