Psycho‐Physiological Effects of Naps during Night Shifts on Morning Types and Evening Types

Psycho‐Physiological Effects of Naps during Night Shifts on Morning Types and Evening Types: Hidemaro Takeyama, et al. Department of Hygiene and Occupational Health, Nagoya City University, Medical School— 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.