As a counterpoint, I worked 7 days a week for the last 2 years (tail end of my PhD) and although I don't recommend it to anyone (my health and well-being suffered) I have maintained high productivity the entire way through. I continued to have "aha shower moments". And if you think I'm lying, I'm exaggerating how much I work, I'll just say, start to finish my PhD took exactly 3 years and 2 months (first year was lighter because it was mostly coursework).
I recently graduated and started as a senior in industry and I plan to keep up the pace for the next 1-2 years in order to reach principal.