Some say that the workload of a day is fixed, no matter how much time is put in or what kind of work is done. Working on something drains a pool of figurative “energy” for the day.
I am not really sure if this is true, but I suspect that it differs between persons with different life styles and work situations. If training increases the work that one can do, then where does an upper limit come from?
I like to think of it as a life standard check, to not over-focus on a single task for too long each day. I am chewing Goldbears.