Introduction: Occupational exposure to biological agents affects a wide variety of occupations, with the health sector being one of the most exposed, especially to HIV, HBV and HCV. At present, in the health sector there is a growing trend to change the 7-hours shift to the 12-hours shift. Method:The objective of this study is to know the influence of 12-hours days on the occurrence of accidents with biological risk in nurses and nursing assistants of hospitals. It is a retrospective cohort study of biological accidents that occurred in 3 public hospitals between 1/1/2017 and 31/12/2018.Results: The incidence of accidents was higher in workers with 12-hours shifts (3,6 per 100 workers), in nurses (4,8 per 100 nurses) and in men (4,1 per 100 men). In the hospitalization wards, the surgical and emergency areas, the incidence of accidents was higher in workers with a 12-hours shift. Conclusions:The realization of 12-hours shifts in the hospital environment among nurses is associated with a higher occurrence of biological accidents. Not assuming a statistically significant risk in hospitalization, surgical area and emergency area, but a protective factor in ICU. Therefore, more studies will be needed to assess the influence of the length shift of work on the health of health workers.