I think we're deviating from the point I was trying to address. The length of the shift isn't relevant, what's relevant is that different patients need different amount of times, and can arrive at any time during a doctor's shift. Based on both of those variables, a single patient can easily require care beyond a single doctor's shift, so saying "getting done with your current patient and then leaving should never lead to a 12-hour shift" doesn't really make sense, since that "current patient" could have arrived during hour 6 of your shift and then required a 6 hours of attention before being discharged. If you believe that doctor-patient continuity is more important than a doctor's rest, then you can easily justify any shift length up to the point where the doctor falls over.