I've read a story somewhere about the manager of an engineering department dealing with critical systems at NASA during the space race. He imposed 9 to 5 work days, as part of his plan to promote a healthy routine. He noticed that overwork leads to mistakes and that nullifies any productivity gain made during extra hours.
It is absolutely true. For work in the trades (machinists, welders, etc), we see the majority of accidents occurring in the final 2 hours of a 12 hour shift. It is naive to think that white collar people don't suffer from fatigue too. The CDC even hosts a study titled "Overtime and Extended Work Shifts: Recent (not so recent now) Findings on Illnesses, Injuries, and Health Behaviors" showing that there may be profound effects on the long-term health of workers as well.