I think you're conflating working hard with working long hours. Truly difficult intellectual work requires a lot of downtime, thinking at the back of your head

I think you're conflating working hard with working long hours. Truly difficult intellectual work requires a lot of downtime, thinking at the back of your head while you're doing other things. If you talk to a professor and ask them about productive intellectual work they'll tell you they don't have as much time for that as they'd like, because they're busy writing grant proposals :)