I tend to loose my edge after the 12-hour mark, so I don't work double shifts. I haven't noticed a change in my effectiveness or made any major errors after 8 hours. (My worst bumple since working 12-hour shifts came at the beginning of the shift after being off the day before.).
I think it also depends on the lifestyle. If someone has worked a 12 hour shift, gone home to tend to kids and went bed after midnight and is doing another 12-hour shift, then that's not good.
I don't do anything between 12 hour shifts but come home, cook, eat, and go to bed early (and play a little on allnurses.com

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It works real well for me to have 4-days off to pursue my BSN and still have a life.
I'm not saying it's easy, working 12-hours nonstop on my busy unit nearly kills me some days.