The present study was designed to describe sleep length and subjective ratings of sleep quality for workers on different shifts in a three-shift system, and to relate inter-individual differences to possible causative factors. 390 workers on three-shift, two-shift and day work systems filled out a questionnaire on work hours and well-being. For three-shift workers sleep quality was best and sleep length longest when the subjects were on the afternoon shift, followed by the morning shift, and then the night shift. Two-shift workers gave ratings almost identical to those of the three-shift workers on corresponding shifts. With increasing age and experience of shift work, sleep quality and sleep length were reduced. Diurnal type was highly correlated with age. Background variables like marital status, number of children and housing conditions did not have any predictive power, and neither had neuroticism or extroversion. Above the age of 45, sleep quality and sleep length in connection with the night shift decreased with increased experience of shift work. It is suggested that the increased night shift difficulties with increasing age and experience may be related to changes in circadian physiology and speed of recuperation.
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