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.
Do you have data on this? It's not something I've observed at any of the small number of Silicon Valley tech companies I've worked at.
It's tempting to think that because Silicon Valley is a center of growth for the economy, if things are done a certain way in Silicon Valley, that's the best way to do them. But Silicon Valley has lots of unusual stuff going on. Some things hurt, some things help, and on balance the unusual stuff seems to help more than it hurts. Also, different companies do things differently.
Are these companies simply wrong and are actually hurting themselves by overextending their human resources? Or does the 40-hour week have exceptions?
See this Quora question answered by people who ought to know: https://www.quora.com/What-are-the-optimal-working-hours-in-an-early-stage-startup-in-terms-of-hours-per-person-per-day-if-you-want-to-maximize-chances-of-success Possible sampling bias: people who are busy working long hours successfully didn't answer the question.
My suspicion is that 40+ hour workweeks are possible given the right kind of work and motivational structure: http://blog.nickwinter.net/the-120-hour-workweek-epic-coding-time-lapse Note that in the post, Nick Winter describes enjoying his work. My guess is that you should work on finding your work enjoyable and energizing before attempting to crank up the number of hours you work.
The claim that people can spend a ridiculous number of hours playing computer games is less controversial, and computer games often have significant intellectual content. If doing your work was hedonically similar to playing a computer game, I imagine that'd make it easier to work long hours. I've also found myself to be more creative and efficient when I find my work enjoyable--I work both harder and smarter.
I haven't solved the problem of finding my work enjoyable all the time, but if you don't have any ideas on making this possible, maybe try reflecting on your aversions and internal motivational structure. I've had success with listening to calm.com while working and trying to notice when more than one thing is competing for my attention and cutting it down to just one, for instance.
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?