The NHS shift system could be putting doctors and patients at risk, experts have warned.
Long hours have been linked to more medical errors
The European working time directive cut junior doctors' hours to a maximum of 13 a day, followed by an 11-hour break.
But UK hospitals then reviewed shift patterns, and as a result many trainees were working 91 hours over seven consecutive nights, the experts said.
Writing in the British Medical Journal, experts from the Royal Free Hospital, London, warned doctors were exhausted.
Led by Roy Pounder, professor of medicine at the Royal Free, they said: "The directive aims to reduce working hours in order to improve workers' health and safety, but the current NHS shift system could threaten doctors' and, moreover, patients' safety."
They pointed to a survey carried out in December 2004 by the Royal College of Physicians (RCP) which found that most trainee doctors in NHS trusts were forced to work a 91-hour week as a series of night shifts.
The team, which also included the president of the RCP, Professor Carol Black, added: "These doctors are exhausted - 70% of specialist registrars in one hospital, working the seven consecutive night shifts, slept for an average of two hours per night while contracted to work, and most had problems with sleep during the daytime."
Recent studies in the US have shown trainees working between 77 and 81 hours a week caused 36% more serious medical errors than those working around 65 hours per week, they said.
"All these adverse effects owing to exhaustion can be expected among British junior doctors forced to work a 91-hour week as a series of night shifts," the UK researchers said.
They said any shift system should have as few successive night shifts as possible, up to a maximum of three.
Professor Pounder and his colleagues suggested the NHS should follow the example of the aviation industry, which has introduced set sleep periods for crew flying overnight.
The researchers said it was inevitable that some doctors would have to work overnight, but said they should be given advice about how to cope.
They added: "The NHS must now reassess the practice of shift work to maximise doctors' safety and efficiency, and to safeguard the interests of patients."
They added: "Those who arrange junior doctors' working schedules should put patients' and doctors' safety first and foremost.
"It is ironic that the working time directive, introduced to protect workers' health and safety, should have led to the imposition of 91-hour nocturnal working weeks for most trainee doctors."
Simon Eccles, chairman of the British Medical Association's junior doctors' committee, agreed that working nights for a week at a time could have a detrimental effect on junior doctors' performance and decision-making, and on patient safety.
Dr Eccles backed the BMJ report's conclusions but urged trusts to stop removing doctors' on-call accommodation.
"Having somewhere to relax on a long shift means you are better rested when you see patients."
He added: "The problem is not necessarily the working time directive itself, but the way hospitals have responded to it."
NHS Employers deputy director Alastair Henderson said the paper raised important issues.
But he added: "The shift patterns that result in junior doctors working 91 hours over seven consecutive nights are legally acceptable under the European Working Time Directive and the hours do balance out to no more than 56 hours a week over a period.
"However, we recognise that these shifts may not be the best way of addressing the balance between service requirements and the needs of junior doctors."
A Department of Health spokesman 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."