Most people know that a person's metabolism slows down as they age. The cruel fact is that even if you keep your food intake steady, unless there is an accompanying increase in exercise, it is likely that you will put on weight. Eating the same amount of food no longer is enough of a strategy to maintain a desired appearance.
I think the same thing occurs with people in the workplace. Their metabolism equates to energy levels, and it is at its highest when the employee first begins a new job. After a person ages in the position, their work metabolism begins to slow. Things become routine. Expectations are raised. The newness wears off and the numbness sets in. Something different has to happen for the person's performance to continue at its desired levels.
Whether planning an exercise plan as you grow older or a performance plan as you become seasoned in your job, keeping the concept of metabolism in mind will help you know when you need to increase the effort just to maintain. It's a great goal to work smarter, but, unfortunately, you'll need to work harder, too.