抄録
An experimental study was carried out under simulated shift-work schedules to examine the effects of a nighttime nap on task performances and psycho-physiological functions on morningness (M-types) and eveningness (E-types) subjects. Sixty male students, classified as M-types or E-types by the Japanese version morningness-eveningness questionnaire, were recruited for this study. Five moderate M-types and eight moderate E-types were selected. Their MEQ scores were 32-41 (average 36.9) in E-types and 60-64 (average 62.4) in M-types, respectively. Experiment periods were simulated shift schedules of 9 consecutive days consisting of 2 d shifts (working 8:00-16:00), 3 night shifts (working 22:00-8:00) and 3 d shifts. They carried out two series of experiments. In one series the subjects took a nap from 2:00 to 4:00. In the other series they did not take a nap during the night shift. In order to minimize the effect of the previous experiments, a rest period of more than one week was inserted between the nap-condition and the no-nap condition experiments. During the shifts, the subjects repeatedly performed two kinds of tasks: typing figures listed randomly on sheets into a computer for 20 min; and after a 5 min break performing mental arithmetic with two adjacent figures, listed randomly on the sheets, for 15 min. During the breaks, psycho-physiological functions and questionnaires on subjective feelings of fatigue and index of the state of anxiety were examined. Throughout the experiments, heart rate variability was also recorded with a portable recorder to evaluate autonomic nerve activity. Task performances decreased and subjective feelings of fatigue and anxiety scores increased in proportion to the length of time worked in both M-types and E-types who had no nap. In M-types, these changes were significantly suppressed by the nap on the first night of duty. Parasympathetic nerve activity for M-types estimated from heart rate variability between 4:00 and 6:00 under nap conditions decreased significantly compared with that under no-nap conditions. Overall changes for E-types, however, were smaller than those for M-types in terms of task performance and psycho-physiological parameters. The present study suggested that taking a nighttime nap was an effective way to reduce adverse effects due to first nocturnal work, especially for M-types.
