I'd say I was much less productive. Usually when we were pushing 80 hours weeks, we were getting ready for a big conference that our bank put on - and cranking out loads of monotonous pitch books that didn't require too much mental energy. At least, not too much if you'd done similar ones 100 times before!
If I had had to create a financial model from scratch - or put together something really complex at hour 80 - that would have been a MUCH harder task.
Makes sense! This I can understand. Analogously, at the end of a software project you tend to have to take care of all of the little things - re-run all of the tests, figure out the packaging script, fill out paperwork, make sure the website is ready, etc. This sort of stuff can be done on relatively little sleep with few breaks. I was imagining the interns putting together models and doing lots of calculations in that 80th hour - seems like sooner or later this would be disastrous.
If I had had to create a financial model from scratch - or put together something really complex at hour 80 - that would have been a MUCH harder task.