The relationship between sleep and shift system, age and chronotype in shift workers

The purpose of this study was to investigate the effects of shift system, a ge, and chronotype on the sleep habits, sleepiness and catnaps of shift wor kers during night work. We administrated a Morningness-Eveningness Question naire and Life Habits Inventory to 561 male shift workers of three differen t shift systems in Korea. The mean scores on the Morningness-Eveningness Qu estionnaire clearly shifted toward the Morning type from the young to old g roups. The waking and bedtimes during the day and evening shift were earlie r for the older than for the younger groups, and these times were earlier a cross the Evening, Intermediate, to Morning type as well. Sleep length duri ng the day shift was longer from the young to old, and it was shorter from the Evening, Intermediate, to Morning type. In the weekly rotation full-day 3-team 3-shift system and continuous full-day 3-team 3-shift system (which had earlier starting times for day shift) the sleep length of the younger workers was short. For the aged workers, sleep length during the night shif t was short because they woke up early. In the weekly rotation full-day 2-t eam 2-shift system that had a short working interval, the reduction of slee p length during night shift was greater than that of the other shift system s. However, the scores on the Morningness-Eveningness Questionnaire showed a significantly negative correlation only in the bedtime during day shift a mong the sleep habit parameters for the independent variable by age. For th e percentage of subjects that reported 'become sleepiness', the degree and time of sleepiness during the night shift were greater and earlier for the older than for the younger workers, and greater for the Morning than for th e Evening type. The percentage of subjects who took a catnap during night s hift, as well as the length of their catnap, were also higher and longer fo r the older groups, and higher and longer for the Morning type than for the Evening type. From these results, we surmised that the sleep habit paramet ers were influenced the age or shift systems rather than the chronotype. Th e sleep length during the night shift was shorter for the aged than for the young. In the shift systems that have a short working interval, the noctur nal was remarkably shorter for the young than the aged. The aged had not ad apted their sleep for the night shift compared to the young, this suggested by the aged's strong complaint of sleepiness and by the fact that a larger percentage of aged subjects said they took catnaps during night shift comp ared to the younger workers.