This isn’t true and there’s no evidence of it being true. Total productivity doesn’t drop as a function of hours worked until somewhere between 40 and 60 hours a week[1]. We know that creativity is more or less constant in individuals so those who produce more do more good work[2].

[1]http://ftp.iza.org/dp8129.pdf

The Productivity of Working Hours, John Pencavel

[2]https://pdfs.semanticscholar.org/abb5/02a8b01b50790a44cf5438...

Creative productivity: A predictive and explanatory model of career trajectories and landmarks.

Dean Keith Simonton

For a popular audience

https://jamesclear.com/equal-odds