Interesting... I did <i>exactly</i> the opposite: after we started working on our startup I realized that I'll just burn myself to death by coding my ass off and not seeing anything outside of my monitors. <p>Besides, you <i>really</i> do start thinking more clearly when you add some physical activity into the mix. Perhaps there may be a simple medical explanation for this, possibly improved blood flow or something.<p>So I started running 3 miles a day, followed by a series of pushups and crunches, every day without exceptions, and actually I am getting close to be in the best shape of my life. :-)
I found the opposite with grad school: I was exhausted from all the work, and consequently I've been inactive and not eating well. Over the summer I'll be back at a normal 9-5 job, and I'm really hoping to get some exercise and nutritional balance in order.
This reminds me of stories of game programmers that would wait 2+ hours for a full compile. In the meantime, they did crunches and curls, and ended up with rock hard abs.