Whenever I hear people talking about "working hard" I try to imagine what they mean.
During my 20s I would often work long hours. A minimum of 12 hours per day. In the most extreme periods perhaps as much as 15 to 17 hours per day. Which was of course hard, but in retrospect, not all that productive. I'd get tired and unfocused, but I would force myself to stay at the office. Sometimes probably doing more harm than good to the projects I was on -- and almost always feeling terribly stressed out.
In contrast, around 2004/2005 I experienced a period of immense productivity while only spending about 5 hours per day at the office. Often less. I'd get in late, focus only on what I worked on for as long as I could and then I would leave the minute I started to feel my concentration going. I rarely even read private email while at work. And I tried to avoid any recreational web browsing. Still, I got a lot more done than when I spent a minimum of 12 hours per day at work.
Not only that; solutions to problems tended to pop up in my mind with much more ease when I made sure I was well rested and spent more time outside the office. I had a physics teacher many years ago who told me that when studying physics he would have a traditional old blackboard in his studio appartment. Every day he would solve one or two problems on it, but never more than that. He said that by limiting himself to one or two problems per day he made sure that he was always hungry for more. He was always looking forward to solving new problems the next day.
I experienced the same feeling when I only allowed myself to work while concentrated and forced myself to go home when I wasn't making progress. It made me much more eager to go to work the next day, and it set my mind abuzz with thought processes while I was riding a bicycle or snapping pictures outside.
The problem I have with the term "working hard" is that it implies that you should be working close to your physical limits. Which, at least in my line of work, seems to be exactly the opposite of what you should do. Because it isn't sustainable. And it doesn't produce good results. It can only be used to impress clueless managers.