That's strange as you would expect quality to suffer in every single respect when forced to work inhumane conditions. I would find a different mechanic if I thought the one I was considering was miserable from being overworked.
Was there an explanation for why outcomes improved? Perhaps it is necessary to look into who conducts these studies as I imagine someone up the ladder profits greatly from having 1 doctor do the job of 2.
One simple reason might be this over: people make errors over time, but they become familiar with the case. That familiarity is lost whenever they hand over a patient. And you can't solve that with documentation since people would be busy all day doing paperwork instead of caring for patients.
This problem exist in every domain, and in medicine it can have deadly consequences. People sign up to become doctors knowing that they will have to work crazy shifts. But I guess they underestimate how the system hobbles them since it is focused on being efficient, not on achieving the best outcome for the patients.
Quality does suffer as the hours drag on, but quality suffers more when you hand off a case (which can be quite complex) to someone else. The tired but experienced-with-this-case doctor still does a better job than the rested doctor who is brand new to the patient's case.
