The large quantity of junior doctors' working hours, equating the effect of long shifts to drinking a few cocktails, study says.
US researchers found 90-hour weeks impaired performance in the same way as alcohol.
UK doctors now work 56-hour weeks but, in the past, working weeks of up to 90 hours were common.
The Journal of the American Medical Association study said it was important to ensure doctors had adequate rest.
It is known that both sleep deprivation and alcohol consumption can impair a person's reaction time, attention, judgment, control and driving ability, reports BBC.
According to Reuters, a survey of resident doctors also found that they were three times more likely than average to have been involved in a motor vehicle crash.
New rules enacted in 2003 lowered the weekly work schedules for U.S. doctors-in-training to a maximum of 80 hours, the report said.
"Residents must be aware of post-call performance impairment and the potential risk to personal and patient safety," study author Todd Arnedt of the University of Michigan, Ann Arbor, wrote in this week's issue of the Journal of the American Medical Association.
"Because sleepy residents may have limited ability to recognize the degree to which they are impaired, residency programs should consider these risks when designing work schedules and develop risk management strategies for residents, such as considering alternative call schedules or providing post-call napping quarters," he wrote.
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