I link here an article from the Guardian by Oliver Bukerman, in Italy reproduced by Internazionale dedicated to the quantity of creative work that can be done every day. He, in turn, takes inspiration from the Alex Pang's book Rest: Why You Get More Done When You Work Less.
The original article can be found here. The Italian version here.
Maybe four hours is too low but, at the same time, pretending to be creative more than four ours a day is not a sin of pride ?
I would be clear, I am not suggesting to my Ph.D. students to work lazily, but to be conscious that quality counts more than quantity and quality also depends on rational management of our own mental health which is the what we need to preserve. Then it is clear that some (short) periods of life require exceptional efforts. But this works only if these are the exception. Not the rule.