The question depends on the kind of work. If you were welding bolts on an assembly line, a measure of how hard you worked is the number of hours you spent doing this work.
On the other hand, if you were a writer, or a chess grandmaster, after 4 hours of intense work, you would notice how your brain tires from the intense concentration and craves for rejuvenation. Working any longer would only produce sub par results. A sensible measure of how hard you work includes how you rest and recuperate for the next day of intense work.
In our own work lives, we lie somewhere on the spectrum between the mindless, cumbersome welding of bolts to the cerebral, creative work of a writer. Yet, we still measure our work with the number of hours we spend at a desk, without accounting for rest and recuperation.
In a creative profession (and we are all in creative professions), to work hard is also to rest hard.