I've read that working excessively long hours hurts productivity. However, I also know that a large majority of people do not truly love their job. So, if these studies sample randomly from the population, only a small proportion of those sampled would truly love their job. I know from personal experience that if I do something I don't really like for a long period of time, I will shut down mentally and the productivity will suffer.
I'm wondering if there are any studies out there that look at the relationship between output and hours for people that are incredibly passionate about what they're doing?