From feedback I've had I'm generally regarded as pretty good at my job and highly productive but my current working hours leave me completely drained, to the point I go home and collapse on the sofa, make dinner and go to bed. And that's off an 8.5 hour day. It's at the point now where it's extremely likely I'll leave software for good.
Software to me is extremely mentally demanding and draining, it requires short burts of extreme concentration, but what is more draining is making up the remaining 4-5 hours looking busy. My experience of software work is that it doesn't scale linearly with number of hours. Unless you reach flow which is extremely rare in a business setting (thanks open-office / scrum) you can get the work done very quickly and then have large periods of unproductive time.
It's my experience that at most 2-3 hours is productively spent, the rest is wasted.
I meet software developers who advocate for, or don't mind, 8, 9, 10 hour days and I simply don't understand it, it's alien to me. Are they being productive all that time?
Edit: to clarify (since I've posted on this topic elsewhere today) the 8.5 hours include a 1 hour lunch break and Fridays are slightly shorter.