This is interesting; my model predicts the exact opposite.

My model is that mental work does not make you tired, it makes you frustrated. There is a difference, because "tired" is related to how difficult the work is, while "frustrated" is more context-dependent -- a simple but time-consuming meaningless task can be very frustrating, while solving a difficult but interesting problem, if you believe that you can do it, is not frustrating. Even expecting a work with unpleasant connotations can make you feel tired.

When I have a lot of free time and something very interesting to do, I can do it for hours and feel happy, at the evening I go to bed thinking about the task, and the next morning I jump out of the bed looking forward to the task. On the other hand, when the task is unpleasant, I try to avoid it by whatever means possible, and in the morning I can't get out of the bed. At least for myself, when I have no problem working, I am happy; when I am avoiding work, I am unhappy.

Of course a larger context can change this. A person happy at work could be using the work as a way to escape from real-world problems. The problem is not with being productive at work per se, but with failing in the larger context.