I have worked for many years as a software-developer and found that more than 5 effective hours per day are impossible or at least not sustainable. 30-32 hours

I have worked for many years as a software-developer and found that more than 5 effective hours per day are impossible or at least not sustainable. 30-32 hours per week is perfect. More is just useless. I have heard that this seems to be true for most intellectual work that requires concentration (5 hours daily max). No sources on that, though.