The study, also published online in the New England Journal of Medicine, further found there wasn't widespread educational or personal dissatisfaction among the residents.
More than 150 hospitals participated in the FIRST study, including several in the Baltimore area, to ensure a large-scale look at the long-debated issue.
Supporters of long hours say they better train residents and limit patient hand-offs that can lead to miscommunication and harm. Critics say exhausted residents are more likely to hurt patients and themselves.
Hours were limited in 2003 and again 2011 by the Accreditation Council for Graduate Medical Education, the accrediting and standards-setting body for about 9,500 U.S. medical residency programs. First-year residents' hours were nearly halved from 30 consecutive hours to no more than 16 hours. The findings could spur the council to revise the hours again.
The study and another looking at general residency programs raised alarms by watchdogs at the Public Citizen's Health Research Group and the American Medical Student Association, which asked federal authorities to intervene to protect residents and patients. They also said failure to alert patients about the trials was unethical. Tuesday, the group said the findings by hospital researchers were "self serving."