The best predictor of how much a knowledge worker will accomplish is not the hours he or she spends, but the days.
The twelve-hour days don’t accomplish any more than the eight-hour days.
I find this to be very true in my own life and career. In fact, I tend to leave the office earlier than anyone else I know... and with all due humility I'll gladly compare my accomplishments and contributions with just about anyone.
Here's the thing - I'm an early bird, and I know my producivity, creativity and overall ability begins to drop off in the late afternoon. If I stick around the office for a few extra hours, the office is getting the least productive part of my day. Plus, it's keeping me from being with my family, which makes me itchy and hurts my morale & motivation. Those extra hours end up being a net loss, not a gain.
So - if you want to get the most out of me, let me leave early. Focus on the days, not the hours. And frankly, I'm writing that as a reminder to myself, as I get ready to head in to a new job in DC. I'm telling myself that not only is it ok to not work long hours... it's better for everyone if I don't.