It is my impression from reading the book "Founders at Work: Stories of Startups' Early Days" and other biographies of accomplished people is that there WERE critical times in their lives where they were obsessed with work. Hours per day worked may be difficult to define. When Einstein was at the peak of his career, he was probably thinking about physics, at least subconsciously, for much more than 8 hours a day.
Random thoughts:
1. There is a difference between doing your own creative work because you have a passion for it and cranking away on something straightforward because your boss told you to.
2. Restated: people cranking out lines of code for e-commerce apps are doing this so they'll get paid. Einstein was thinking about physics simply because he wanted to.
3.
This paper is peer-reviewed and says a lot of the same stuff.
Anecdote alert: my personal experience has taught me that dull repetitive jobs (even high-paying knowledge-type jobs) make me feel dumb and uncreative. YMMV.
