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 durin g weekends with 8-hour shifts during weekdays with respect to sleep, sleepi ness, physical effort, and performance. Methods Thirty-one subjects at a power plant participated. Sleep, sleepines s, and physical effort were measured with a diary. About half of the subjec ts 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 nigh t shift than on the 8-hour night shift. However, the subjects who had the s ame 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 wa s 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 w ith respect to performance. Conclusions It was suggested that the difference in sleepiness between 8- a nd 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 t han to shift duration. Thus the most likely conclusion is that 12-hour shif ts do not cause increased sleepiness or impaired performance or disturbed s leep.