Introduction. The aim of this study was to evaluate the effects of consecutive night shifts (CNS) and shift length on cognitive performance and sleepiness. Materials and methods. This study evaluated the sleepiness and performance of 30 control room operators (CROs) working in 7 nights, 7 days, 7 days off (7N7D7O) and 30 CROs working in 4 nights, 7 days, 3 nights, 7 days off (4N7D3N7O) shift patterns in a petrochemical complex on the last night shift before swinging into the day shift. To assess cognitive performance, the n-back test, continuous performance test and simple reaction time test were employed. To assess sleepiness, the Karolinska sleepiness scale was used. Results. Both schedules indicated that the correct responses and response times of working memory were reduced (p=0.001), while intentional errors and sleepiness increased during the shift work (p=0.001). CNS had a significant impact on reaction time and commission errors (p=0.001). Conclusion. The main duty of CROs at a petrochemical plant is checking hazardous processes which require appropriate alertness and cognitive performance. As a result, planning for appropriate working hours and suitable number of CNS in a rotating shift system is a contribution to improving CRO performance and enhancing safety.