That would be Desai, et al. Though I can't access the paper right now, they say that the 2011 regulations have tripled handoffs, increasing handoff risk. It also turned out that the average amount of sleep per week was not increased by much after instituting the regulations.
[2] Desai SV, Feldman L, Brown L, et al. Effect of the 2011 vs 2003 Duty Hour Regulation–Compliant Models on Sleep Duration, Trainee Education, and Continuity of Patient Care Among Internal Medicine House Staff: A Randomized Trial. JAMA Internal Medicine, 2013; DOI: 10.1001/jamainternmed.2013.2973
I would argue that the literature is basically worthless here, because they're mostly looking at modulations around an absolutely ridiculous baseline. The 2003 changes dropped hours DOWN to 80/week and 16 or 28 per shift.
As an analogy, imagine reducing your cheeseburger intake from 60 per week to 40 per week: it probably won't have a huge impact on physical fitness.
The hand-off issue is also weird because it's at least theoretically improvable, whereas there's no real way (barring go-pills) to reduce sleep-related issues.
That's not really what that study says. The paper finds no difference between a "flexible" schedule and the standard duty-hour one.
Neither one of these schedules is really "reduced" compared to any sort of typical level: they're both ~80 hrs/week (and probably more).
From the paper:
"Programs assigned to the flexible-policy (intervention) group were required to adhere to ACGME duty-hour requirements of limiting work to 80 hours per week, 1 day off in 7 days, and on-call duty no more frequently than every third night, but they were granted a waiver by the ACGME to waive four duty-hour requirements (from the 2003 and 2011 reforms) concerning maximum shift length and minimum time off between shifts (to facilitate continuity of care) (Table 1)"
[1] https://www.sciencedaily.com/releases/2013/03/130325183819.h...
[2] Desai SV, Feldman L, Brown L, et al. Effect of the 2011 vs 2003 Duty Hour Regulation–Compliant Models on Sleep Duration, Trainee Education, and Continuity of Patient Care Among Internal Medicine House Staff: A Randomized Trial. JAMA Internal Medicine, 2013; DOI: 10.1001/jamainternmed.2013.2973