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To investigate the effects of transition from 8- to 12 hour shifts on sleepiness and vigour of employees during morning and night shifts.Thirty-nine airline ground crew shift workers (mean age 38.9±8.2 y; mean seniority 13.9±7.1 y, 19 male) volunteered in a pilot study. During each round, employees were assessed during one morning and two night shifts, by completing a sleepiness questionnaire (the Karolinska Sleepiness Scale, KSS) hourly, and a vigour questionnaire (3 times in the 8 hour, 4 times in the 12 hour rounds) per shift. Repeated measures ANOVA was performed to compare changes in sleepiness and vigour levels in 8 hour and 12 hour shift plans, during day and night shifts.For morning shifts main effects were found for sleepiness [F(3,58)=13.1, p<0.001] and for shift plan [F(1,18)=7.4, p<0.014]. Sleepiness levels rose throughout both morning shifts, and were lower during the 12 hour than during the 8 hour shifts. Furthermore, main effects were found for vigour [F(1,25)=11.37, p<0.001] and for shift plan [F(1,18)=14.87, p<0.001]. Vigour levels decreased in both morning shifts, and were higher during 12 hour shifts than in the 8 hour shifts. No interactions were found.For night shifts main effects were found for sleepiness [F(7,18)=9.9, p<0.001], and for sleepiness*shift plan [F(3,81)=3.2, p<0.023]. During the 8 hour round, sleepiness consistently increased with a slight decrease towards the end of the shift, while in the 12 hour round sleepiness increased until 04:00 am, after which there was a decrease and again and increase towards the end of the shift.Furthermore, main effects were found for vigour [F(1,25)=11.37, p<001]), and for shift plan [F(1,18)=14.87, p<0.001], during the 12 hour shift round participants’ vigour levels were higher than in the 8 hour shift round. Vigour levels decreased during the night in both rounds, no interaction was observed.An overall improvement was observed in somnolence and vigour indices of employees after the transition to 12 hour shift, which appears to be preferable over the 8 hour shift.