Dr. Srijan Sen of the University of Michigan Health System in Ann Arbor said the percentage of study participants who met criteria for depression increased from 3.9 percent before interning to an average of 25.7 percent during the internship.
The researchers studied 740 interns entering residency programs in 13 U.S. hospitals in 2007 and 2008 who completed five secure online surveys during the course of their internship. Sixty-three percent also provided saliva samples for genetic analysis.
The report, published in the Archives of General Psychiatry, linked a number of factors to increased depression symptoms -- including long work hours, genetic variation and difficult early family environment.
"We know that internship is a time of high stress," Sen said in a statement. "While doing my internship, it was clear to me that even people who seemed to be well adjusted during our initial orientation started to struggle as the internship progressed; never smiling and they were having difficulty with sleep and losing or gaining a lot of weight."
The findings are published online ahead of print in the June print issue of Archives of General Psychiatry.
2014: The Year in Music [PHOTOS]