Don’t check into a hospital in July. That’s according to a recent study by California researchers, who studied over 62 million death certificates from 1979 to 2006. They found a 10 percent increase in deaths linked to medication errors in counties with teaching hospitals, but not in other counties.

This has long been referred to by physicians as the “July effect:” a spike in medical errors when new residents join hospitals. Residents can put in long hours – 30-hour shifts, for example. According to a professor of sleep medicine who did a survey of public’s attitudes toward medical residents’ work hours, “one in five resident physicians admits to making a fatigue-related error that has injured a patient, and one in 20 admits to making a fatigue-related error that has resulted in the death of a patient.” He said that “Working for 24 hours without sleep impairs performance to a degree that is comparable to being legally drunk.”

The Institute of Medicine has concluded that residents should work no more than 16 hours straight. Of course, that would drive up costs – $1.7 billion annually, according to the IOM. The Accreditation Council for Graduate Medical Education is expected to issue its own recommendations later this month.