Do you know of any studies that address these issues?
In a sense, Silicon Valley is an ongoing study. Here you have a very large number of firms in a competitive environment, with low cost of entry, each firm looking for a productivity edge over their rivals. If holding the work-week down to 40 hours really made their employees more productive, don't you think you'd see lots of successful firms that have tried it?
And yes, I'm aware that there's signalling going on, but you'd still expect to see some successful firms that worked like that. Moreover, Silicon Valley firms go just the other way, doing everything they can to keep employees in the office by making it "fun" - that's the opposite of what a signalling explanation would predict, but strongly predicted by an "extra time in the office really is productive" theory.
Increasing the work week generally provides a short-term and more easily measured benefit (getting this project done sooner) but long-term and harder to measure harms (higher error rate and higher maintenance cost). Bean-counters are notoriously bad at trading off short-term benefit for long-term harm, and at preferring benefits which are easy to measure.
Conventional wisdom, and many studies, hold that 40 hours of work per week are the optimum before exhaustion starts dragging your productivity down too much to be worth it. I read elsewhere that the optimum is even lower for creative work, namely 35 hours per week, though the sources I found don't all seem to agree.
In contrast, many tech companies in silicon valley demand (or 'encourage', which is the same thing in practice) much higher work times. 70 or 80 hours per week are sometimes treated as normal.
How can this be?
Are these companies simply wrong and are actually hurting themselves by overextending their human resources? Or does the 40-hour week have exceptions?
How high is the variance in how much time people can work? If only outliers are hired by such companies, that would explain the discrepancy. Another possibility is that this 40 hour limit simply does not apply if you are really into your work and 'in the flow'. However, as far as I understand it, the problem is a question of concentration, not motivation, so that doesn't make sense.
There are many articles on the internet arguing for both sides, but I find it hard to find ones that actually address these questions instead of just parroting the same generalized responses every time: Proponents of the 40 hour week cite studies that do not consider special cases, only averages (at least as far as I could find). Proponents of the 80 hour week claim that low work weeks are only for wage slaves without motivation, which reeks of bias and completely ignores that one's own subjective estimate of one's performance is not necessarily representative of one's actual performance.
Do you know of any studies that address these issues?