One argument for a longer work week:
You always have to do a fixed baseline of background work: Answering emails, meetings, filling out the forms for your company dental plan. Say that's 20 hours. So, a 60-hour week is not 50% more quality work-time (coding, etc.) than a 40-hour week, it is 100% more. Of course, it's best to arrange things to minimize the scut-work and focus on the highest-value effort.
Another argument: You want to avoid clock-watching. When you're hyped up, you work longer hours because you have the momentum and the urge to fix that one last problem. If you keep the work-day to 8 hours, it's hard to maintain that sense of drive.
Ideally, you maintain a sense of drive for the 8 hours (going over occasionally as needed) -- if you can manage that, it's the best combination.
These are reflective-decision arguments: Do X to make sure your mind focuses on certain goals. If we were reflectively consistent we wouldn't need to fool our own minds. As with other forms of rationality, you will do better if you can maintain reflective consistency than if you try to hack your way around it.
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?