I think 8 hours is way too much to be productive, AND creative. Not physically, I can work crazy hours, not a problem. If it's physical activity, and requires body, rather than mind.
I did crazy working hours as chef, 8 hours is nothing, unless is creative/ software job.
I am beginner as coder, and Iam working on different field, so I have good comparison between these 2 worlds.
To solving problems, need tons of creativity, and the long hours are enemies of creativity.