Observation: the relationship between the number of hours you work and how "hard" you work is non-linear. This observation ignores presenteeism (which itself heavily distorts the how "hard" people work). To explain, imagine that your job is to assemble cardboard boxes, and each box takes 30 seconds to make, that if you work at 120 boxes per hour consistently, the impact of that upon you physically, emotionally and intellectually increases per hour. Human beings have a finite capacity for productive output within a 24 hour period, requiring rest and sleep (and a break from monotony).
Hypothesis: the graph of "hard" vs hours per day looks something like the following: