My current postdoc position requires that I work 41 hours per week during a 5-day work week. As my first post-graduate position, I decided to take this seriously and actually track my hours. (People at labs have to do this anyway, so I figured it could be a helpful habit.) I found that except for the weeks where I definitively over work, I often can't meet 41 full, productive hours. Accounting rigorously for breaks, I often fall short by a couple hours, and even then, I know that some hours I log as working, I'm usually struggling to focus. Most weeks I feel guilty and make up for the hours by staying late or working over the weekend, which is of course undesirable.
So I wonder: is anyone actually productive all hours of their work week? Is there any research on how to optimize one's mental energy during the work week?
Please refrain from answers like, "You're being too hard on yourself." I'm a freshly minted PhD, of course I'm being too hard on myself.