Comparison of mental performance and sleepiness between 8 and 12 hrs shift works in an oil refinery plant in Iran

There are different types of shift work programs, each of them has its pros and cons. The purpose of this study was to compare sleep and mental performance in 12-hour and 8-hour shift works. In this longitudinal study, 61 workers from 187 shift workers in one of oil refinery plants were chosen. They were supposed to work in a schedule of 12- hour shift works for 6 months and then return to 8-hour shift works again. We used a demographic questionnaire, sleep diary and PVT software in the beginning and the end of the same shift. Data was collected at the end of 6 months of 12-hour shifts and then after 6 months of 8-hour shifts from the same workers. Total sleep time mean in 12-hour shifts was 6.20 hours and in 8-hour shifts was 6.41 hours, but the difference was not significant. The difference between means of reaction times in the beginning of 12-hour shifts (451.80 milliseconds) and 8-hour shifts (416.12 milliseconds) was not significant. The difference between means of reaction times in the ending of 12-hour shifts (382.22 milliseconds) and 8-hour shifts (390.98 milliseconds) was not significant either. The difference between means of false responses in the beginning of 12-hour shifts (3.74) and 8-hour shifts (2.98) was significant. (p<0.05) This difference in the ending of the shifts was significant as well. (3.74 in 12-hour versus 2.36 in 8-hour shifts, p<0.01). Total sleep time in 12- hour shifts is not significantly different from 8-hour shifts. 12-hour shift may decrease mental function (increasing false responses of PVT in the beginning of 12-hour shifts comparing to 8-hour shift). Different sleep patterns may be an explanation for these findings. Key words: 12-hour and 8-hour shift works, Mental performance, Reaction time, False responses.