Many hospitals and nursing homes are endangering patients by allowing or requiring nurses to work more than 12 hours a day, the National Academy of Sciences said on Tuesday. Such long hours cause fatigue, reduce productivity and increase the risk that the nurses will make mistakes that harm patients, the academy said in a new report commissioned by the federal government.

Donald M. Steinwachs, chairman of the health policy department at Johns Hopkins University, said fatigue was a "major cause of mistakes and errors" in hospitals and nursing homes. Mr. Steinwachs was chairman of the panel of 18 experts who conducted the study. The report said many nurses and nursing assistants worked more than 12 consecutive hours, with some working double shifts of 16 hours.

To reduce "error-producing fatigue," the report said, state officials should prohibit nurses from working more than 12 hours in any 24-hour period or more than 60 hours a week. The report, from the academy's Institute of Medicine, said, "Long work hours pose one of the most serious threats to patient safety, because fatigue slows reaction time, decreases energy, diminishes attention to detail, and otherwise contributes to errors."

Many hospitals and nursing homes have too few nurses to take proper care of patients. Intensive care units at hospitals should have one licensed nurse on duty for every two patients, the report said. Nursing homes, it said, should have one registered nurse for every 32 patients and one nursing assistant for every 8.5 patients.

The academy found overwhelming evidence that as levels of nurse staffing rose the quality of care improved, because nurses had more time to monitor patients and can more readily detect changes in their conditions. The academy said the nation's 2.8 million licensed nurses and 2.3 million nursing assistants accounted for 54 percent of health care workers. Thus, said Dr. Harvey V. Fineberg, president of the Institute of Medicine, "It is nurses who deliver most of the care we receive." Pamela Thompson, chief exequtive of the American Organization of Nurse Executives, a subsidiary of the American Hospital Association, said it was "an accepted practice" for nurses to work 12-hour shifts.

The panel did not distinguish between voluntary and mandatory overtime. Ada Sue Hinshaw, a panel member who is dean of the School of Nursing at the University of Michigan, said: "The fatigue effects are the same. Medical errors start climbing after 12 hours of work." To reduce such errors, the panel said, nurses should be more involved in the day-to-day management of hospitals and nursing homes.