A study has shown how important the reduction of junior doctors' hours has been, equating the effect of long shifts to drinking a few cocktails.
Junior doctors often have busy shifts
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.
Fatigue reduction strategies
The researchers from Brown University and the University of Michigan analysed 34 trainee doctors.
They were tested before and after working a month of 44-hour weeks in clinics with no night-shifts.
Tests were carried out before and after they consumed three vodka and tonics within 30 minutes.
In the second session, they were tested on the day after a night-shift at the end of a month of 90-hour weeks on wards or in the intensive care unit, including night-shifts every fourth or fifth night without having had an alcoholic drink.
Similar impairments were seen in the doctors' vigilance, attention, and driving skills in standardised tests after the shorter working weeks plus a few drinks - when they had a blood alcohol count of 0.04% (half the legal limit in most US states and all of the UK) - and after 90-hour weeks with no alcohol.
Sleep diaries and automated wristbands, which responded to movement, were used to assess how much rest the doctors had.
Those who worked longer weeks had significantly less sleep per night than those working the lighter shifts - even when they were not on duty overnight.
Todd Arnedt, a sleep psychologist at the University of Michigan Medical School, said: "This adds to the growing evidence that sleep deprivation among medical residents [junior doctors] significantly impairs their ability to perform, although it is important to note that we did not assess performance on specific medical tasks.
"Our study, like others before it, does raise concerns about the performance of sleep deprived physicians-in-training and suggests that strategies aimed at reducing fatigue-related impairments are likely necessary."
Dr Arnedt added: "We need to continue to find simple, practical, and effective strategies that hospitals and senior doctors can take to reduce sleep deprivation among residents."
Since the study was carried out, the US has introduced new rules to limit the average working week to 80 hours, and maximum shift-length of 24 hours.
Doctors in the UK work an average of 56 hours each week.
But the British Medical Association warns doctors can spend longer at the hospital site because of the way their shift patterns are designed, meaning they might not get the rest they need before going back on to the wards.
Dr Mike Peters, Head of the BMA's Doctors for Doctors Unit, said: "Excessive working hours are bad news for doctors and patients.
"It is difficult for over-tired, stressed and burnt out doctors to give patients the quality of care they need."
A Department of Health spokesperson said: "We believe that junior doctors' working patterns should strike a sensible balance between services designed around patients and services which support
doctors' working lives and their training."