Evan Robinson - Why Crunch Mode Doesn't Work: 6 Lessons:

"Workers can maintain productivity more or less indefinitely at 40 hours per five-day workweek. When working longer hours, productivity begins to decline. Somewhere between four days and two months, the gains from additional hours of work are negated by the decline in hourly productivity. [...] A hundred years of industrial research has proven beyond question that exhausted workers create errors that blow schedules, destroy equipment, create cost overruns, erode product quality, and threaten the bottom line."