Dale Carnegie wrote about guys working in a foundry who, the experts calculated, should be able to carry 47 tons of pig iron per day, yet they were only actually able to carry 12 (or something similarly smaller than 47). Upon closer inspection, Carnegie noted that the workers took no more than a short break for lunch each day, so he advised them to sit and rest for a pre-determined amount of time each hour to renew their strength. As if by magic, the men were all then able to carry the predicted 47 tons per day, leaving employees and management quite satisfied.

Your brain is no different in its need for rest than your body. I've discovered that I need a break at about 3pm every afternoon, so I find a quiet place to lay down, close my eyes, and think about precisely nothing. After ten or fifteen minutes, I am ready to go back to thinking very hard. I've had co-workers who choose to sit at their desks all day looking very diligent rather than rest when they need it, and they are far less productive. (Of course, there are a few out there who don't seem to need to ever rest, and they're much more productive than I.)

You're not being paid to log hours; you're being paid to accomplish something. If you meet or exceed your employers' requirements for you, then they are getting their money's worth.