My PhD, in physics, was a mix. When I was building the physical apparatus itself and plumbing it up, I was working 70 hour weeks because it was more or less mi

My PhD, in physics, was a mix. When I was building the physical apparatus itself and plumbing it up, I was working 70 hour weeks because it was more or less mindless physical labor. When I was writing software to operate the experiment or analyze data, I settled into more of a few hours a day of deep work pattern. Part of that might be the nature of science; after I implemented an idea, it might be hours before the experiment fully responded to my changes or the computers finished crunching numbers. And I needed that feedback to inform what I'd do next. Writing my dissertation was somewhere in the middle. The actual writing required focus, but a lot of it was tracking down references or making figures, which required some thought but not too deep. That's probably the closest I came to a 40 hour week. Based on other replies, it sounds like other people have experienced similar relationships of the nature of the work to their capacity to do it.