New York has become the first state to limit the grueling work hours of internsand residents to reduce the risk of error by exhausted doctors-in-training. Discarding the century-old tradition of 36-hour shifts and 100-hour work weeks for postgraduate traineesin medicine, the state Health Department now limits shifts to 24 hours and the work week to 80 hours. Proponents of the tradition say shunning sleep toughens young doctors to the rigors of medicine and lets them study the progression of a patient’s illness. Critics say bleary-eyed residents endanger patients. The Hospital Assn. of New York State and Blue Cross, backed by the state medical society, have sued to block the regulations, charging that they will be too costly. The hospital industry estimates 4, 500 to 5,000 workers must be hired statewide to implement the plan.