Many people hold the view that "crunch mode doesn't work"; one extreme viewpoint, endorsed by Extreme Programming, is that working more than forty hours per week doesn't result in increases in total productivity for each week. I will call this the "Productivity Laffer Curve" viewpoint.
The "Productivity Laffer Curve" hypothesis is odd, because "work" is a vague term. It's like being told that eating green things reduces your risk of cancer. What sort of green things? Vegetables? Mold? Green M&M's? The analogy here is that, just as there's no reasonable causal mechanism how greenness can directly prevent cancer, there's no reasonable causal mechanism for how the fact that someone labeled a task as “work”, can directly reduce your ability to do that task. If people who work more than 40 hours lose effectiveness, then there must be some causal factor involved, such as physical fatigue, mental fatigue, or loss of motivation; and it may be better to determine what those factors are, and address those factors directly, rather than artificially curtail your work week.
One reason for skepticism about the “Productivity Laffer Curve” is that companies don't always push 40-hour work weeks, even though having a 40-hour work week creates a benefit for the company (it makes it easier to recruit and retain workers) beyond the benefits of greater productivity. Therefore, any company with a 40-hour work week would have an enormous advantage over other companies: the 40-hour work-week company would have a recruiting and retaining advantage, *without* having to sacrifice even a little bit of productivity. However, I don't think we see this in practice.