The aim of this study was to compare permanent 12-hour day and night shifts (shift change over times at 0500 and 1700) in a shift system with 3 work periods followed by 4 free days.
Sleep diaries were collected after main periods of sleep, and sleepiness ratings [Karolinska sleepiness scale (KSS)] were obtained 4 times during the last free day and also during the following 3 workshifts. Eighteen to twenty night workers and 8-10 day workers (depending on the instrument) participated.
The day workers were significantly sleepier during their workdays. Times for going to bed and for rising differed between the groups. The amount of sleep per week did not differ between groups, but the pattern across days did in that the day workers had a short sleep (5 hours) before the first day and 6 hours of sleep after the other two. Night workers slept long (9 hours) before the first shift and had 6.5-hour sleep periods after the other shifts. During free time the day workers slept around 9 hours and the night workers around 8 hours. Sleep quality and ease of awakening showed no group differences in overall levels, but the day workers had difficulties awakening before their shifts. The night workers had little variation in sleep quality or difficulties awakening.
The suggested explanation for the greater sleepiness and difficulties awakening among the day workers was the early start of the shift and the difficulties the workers had with phase advancing their sleep-wake rhythm.