As the 37 Signals Book Rework points out: "Working more doesn't mean you care more or get
more done." According to a recent survey by Microsoft, the average worker is only productive for about 30 hours of their 46 hour work week.
If all you do is work, you're unlikely to have sound judgments. Your values and decision making wind up skewed. You stop being able to decide what's worth extra effort and what's not. And you wind up just plain tired. No one makes sharp decisions when tired.