Lala - I have worked with you but never as an Assistant. You weren't IN the hangar, but adjacent it in the room upstairs.
I can only make judgments on what I have experienced whilst knowing you. Lets say many other people are of the same opinion as me
As for the lack of grey hair, yep, you need some (hair that is).
To answer the original question, working 12 hrs is no different than working 8hrs. If one is good at ones job, then being able to apply yourself over a 12hr period shouldn't be difficult. Like I said before, when you did 12hr shifts, you didn't actually do any work in the first place. Yes your assistants did most of the work, but that wasn't through you wanting to ensure they learn more. :P