I think what it comes down to is focus. I work as hard as I can on the project at hand and (usually) know when it's time to take a break, but the focus remains

I think what it comes down to is focus. I work as hard as I can on the project at hand and (usually) know when it's time to take a break, but the focus remains on the project, not the break. If I cared more about snowboarding, for example, than my startup, I'd be a lot better at snowboarding but I can guarantee the company wouldn't exist. In my mind startups are all about short bursts of intense and admittedly unsustainable bursts of energy. That's why it's a startup with an end goal in sight, not an established company that intends to continue forever. It may make sense for 37signals to fire the workaholics, but I don't think it does for me.