igda, the international game developers association, has an article by Evan Robinson called Why Crunch Mode Doesn't Work: 6 Lessons. It's an excellent rebuttal to any management's attempts at forced overtime (I've heard it called "mandatory voluntary overtime" at one place). Citing numerous studies on workplace productivity, Evan comes up with six lessons.

1. Productivity varies over the course of the workday, with the greatest productivity occurring in the first four to six hours. After enough hours, productivity approaches zero; eventually it becomes negative.
2. Productivity is hard to quantify for knowledge workers.
3. Five-day weeks of eight-hour days maximize long-term output in every industry that has been studied over the past century. What makes us think that our industry is somehow exempt from this rule?
4. At 60 hours per week, the loss of productivity caused by working longer hours overwhelms the extra hours worked within a couple of months.
5. Continuous work reduces cognitive function 25% for every 24 hours. Multiple consecutive overnighters have a severe cumulative effect.
6. Error rates climb with hours worked and especially with loss of sleep. Eventually the odds catch up with you, and catastrophe occurs. When schedules are tight and budgets are big, is this a risk you can really afford to take?

For managers that need a good rule of thumb:

"... at approximately eight 60-hour weeks, the total work done is the same as what would have been done in eight 40-hour weeks."

And my favorite:

"In the short term, working over 21 hours continuously is equivalent to being legally drunk."

[via the ever-vigilant Lasse]