Dead tired: weary doctors function as if intoxicated.

After a month of long hours of challenging work, fatigued physicians show impairments in driving and other tasks requiring constant attention and quick reactions. The changes are comparable to those that the physicians display, when better rested, after downing three or four alcoholic drinks, a new study finds.

During their years of practical training after medical school, many resident doctors spend long shifts on call and, in some months, work more hours than they have off. Professional skills aside, residents report having more auto mishaps than other people do. For years, physicians have debated whether the residents' demanding schedules and sleep deficits have avoidable negative consequences for themselves and patients.

To quantify the effects of long days of intense work on driving and other skills, clinical psychologist J. Todd Arnedt and his colleagues at Brown University in Providence, R.I., tested 34 physicians during their residency training in pediatrics.

In some months, the volunteers worked about 44 hours per week. At other times, most of them labored 90 hours per week and had frequent shifts of up to 36 consecutive hours. Most of the tests were performed before July 2003, when the Accreditation Council for Graduate Medical Education limited residents to an average workweek of 80 hours.

In the study, each resident faced the same battery of tests near the end of a relatively easy month and again after a tougher month that had culminated in an all-night shift. To compare the effects of long work hours with those of alcohol intoxication, the researchers gave the participants three or four shots of vodka, mixed with tonic water and lime, shortly before some of the tests toward the end of an easy month.

In one series of tests, residents in a simulator attempted to drive at 60 miles per hour. When sober and relatively rested, most residents stuck close to the target speed and stayed on the road for the duration of the half-hour drill. After drinking, however, most of them had more trouble maintaining a constant speed and drove off the road at least once.

After working intensely for a month, the same residents also left the road at least once, on average, despite being sober, and drove at significantly more-erratic speeds than they did in the other tests.

In another test series, volunteers displayed reaction times that were slowed in equal measure by drinking or by having worked long hours.

Arnedt, who's now at the University of Michigan in Ann Arbor, and his colleagues report their results in the Sept. 7 Journal of the American Medical Association.

"Clearly, when residents are sleep deprived, they are at increased risk of causing accidents while driving home," comments neurologist Phyllis Zee of Northwestern University in Evanston, Ill. "Limiting hours ... is part of the solution [to] driving drowsy," she says.

However, tired physicians may rise to the challenge of providing competent medical care, she says, and passing duties among physicians on short shifts creates its own risks to patients.