AimsThis study aimed to determine the peak hours of sleepiness and the factors affecting the sleepiness levels of nurses.BackgroundSleepiness is commonly seen in individuals working night shifts. However, in case of nurses, this sleepiness can be a major threat to patient and staff safety.MethodThis was a prospective cross‐sectional study. Data were collected between July and September 2023, and a stratified sampling method was used according to the departments in which the nurses worked. Data were collected using the Personal Information Form and Visual Analog Scale. Nurses reported their sleepiness levels at the beginning of each hour between midnight and 8:00 am.EQUATOR checklistThe study adhered to the STROBE checklist for reporting.ResultsThe mean sleepiness levels of emergency department nurses, intensive care nurses, internal or surgical clinic nurses, and all nurses were 59.75 ± 15.50, 43.53 ± 20.49, 44.67 ± 18.88, and 49.15 ± 19.67, respectively. The highest sleepiness level of the nurses was at 05:00 am. A significant correlation was found between the variables of age, gender, marital status, sleep quality, number of patients cared, working style and satisfaction with working in the department, and sleepiness level (P < 0.05).ConclusionNurses working night shifts experience sleepiness (highest at 5:00 am). In addition, the sleepiness levels of nurses are affected by some personal and clinical factors.Implications for nursing and nursing policyBased on the results, there is a need for necessary policies regarding nurses' working hours and working conditions. To protect patient and employee safety, necessary strategies should be devised regarding the hours when nurses experience the highest sleepiness and the factors affecting sleepiness levels.