Effects of alternating 8- and 12-hour shifts on sleep, sleepiness, physical effort and performance.
OBJECTIVES: The aim of the present study was to compare 12-hour shifts during weekends with 8-hour shifts during weekdays with respect to sleep, sleepiness, physical effort, and performance. METHODS: Thirty-one subjects at a power plant participated. Sleep, sleepiness, and physical effort were measured with a diary. About half of the subjects carried out a reaction-time test during both 8- and 12-hour morning and night shifts. The remaining subjects carried out a vigilance task. RESULTS: Sleepiness was higher and physical effort lower on the 12-hour night shift than on the 8-hour night shift. However, the subjects who had the same level of physical effort on 8- and 12-hour night shifts did not differ with respect to sleepiness. During the 12-hour morning shift, sleepiness was lower and the sleep length was longer than on the 8-h morning shift. The subjects who had the same amount of sleep for 8- and 12-hour morning shifts showed no difference in sleepiness. Sleep did not differ between 8- and 12-hour night shifts. There was no difference between 8- and 12-hour shifts with respect to performance. CONCLUSIONS: It was suggested that the difference in sleepiness between 8- and 12-hour shifts is related to differences in sleep length for the morning shift, and to differences in physical effort for the night shift, rather than to shift duration. Thus the most likely conclusion is that 12-hour shifts do not cause increased sleepiness or impaired performance or disturbed sleep.