Thanks, you're right. What I really meant was something backed by data. This is more about business management practices or motivational psychology and "optimum working hours" must be a well studied topic. There must be more than some weak anecdotal arguments about Sheryl Sandberg or Ford Motor Company in early 1900s.
There was a presentation given by a programmer that looked at the research. I seem to recall that 40 hours was optimal for manual labour but it was actually less for mental work like programming, but the real kicker was it compared actual productivity with self-perceived productivity and claimed that you felt like you were making progress even when you weren't.
This isn't the presentation I was thinking of, but it covers similar ground (and quotes 35 hours as the optimum for brain-based work):
You can check out a few classics on developer productivity: Frederick Brooks' "Mythical man-month" and Tom Demarco's "Slack". As far as I recall, they both go into concrete data about the productivity drop-off that ensues after +40 hours for extended periods of time.
