Psycho-physiological effects of naps during night shifts on morning types and evening types

Abstract: 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 morningnesseveningness 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, psychophysiological 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. Received Aug 27, 2001; Accepted Dec 21, 2001 Correspondence to: H. Takeyama, Department of Hygiene and Occupational Health, Nagoya City University, Medical School, Nagoya, 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 nonap 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. (J Occup Health 2002; 44: 89-98

Effects of the length and timing of nighttime naps on task performance and physiological function

OBJECTIVE: To examine the effects of the length and timing of nighttime naps on performance and physiological functions, an experimental study was carried out under simulated night shift schedules. METHODS: Six students were recruited for this study that was composed of 5 experiments. Each experiment involved 3 consecutive days with one night shift (22:00-8:00) followed by daytime sleep and night sleep. The experiments had 5 conditions in which the length and timing of naps were manipulated: 0:00-1:00 (E60), 0:00-2:00 (E120), 4:00-5:00 (L60), 4:00-6:00 (L120), and no nap (No-nap). During the night shifts, participants underwent performance tests. A questionnaire on subjective fatigue and a critical flicker fusion frequency test were administered after the performance tests. Heart rate variability and rectal temperature were recorded continuously during the experiments. Polysomnography was also recorded during the nap. RESULTS: Sleep latency was shorter and sleep efficiency was higher in the nap in L60 and L120 than that in E60 and E120. Slow wave sleep in the naps in E120 and L120 was longer than that in E60 and L60. The mean reaction time in L60 became longer after the nap, and faster in E60 and E120. Earlier naps serve to counteract the decrement in performance and physiological functions during night shifts. Performance was somewhat improved by taking a 2-hour nap later in the shift, but deteriorated after a one-hour nap. CONCLUSIONS: Naps in the latter half of the night shift were superior to earlier naps in terms of sleep quality. However performance declined after a 1-hour nap taken later in the night shift due to sleep inertia. This study suggests that appropriate timing of a short nap must be carefully considered, such as a 60-min nap during the night shift.<br>OBJETIVO: Para investigar os efeitos da duração e horário de cochilos noturnos sobre o desempenho e as funções fisiológicas foi realizado um estudo experimental por meio do trabalho noturno simulado. MÉTODOS: Seis estudantes foram recrutados para o estudo que consistiu de cinco experimentos. Cada experimento consistia de três dias consecutivos com um turno noturno (22:00-8:00h) seguido por um período de sono diurno e noturno. Os experimentos compreendiam cinco condições em que a duração e o horário dos cochilos eram manipulados: 0:00-1:00 (E60), 0:00-2:00 (E120), 4:00-5:00 (L60), 4:00-6:00 (L120) e sem cochilo (N-nap). Durante os turnos noturnos, os participantes foram submetidos a testes de desempenho. Um questionário sobre cansaço subjetivo e um teste de freqüência crítica de luz foram aplicados depois dos testes de desempenho. A variabilidade da freqüência cardíaca e a temperatura retal foram registrados continuamente durante os experimentos. Polissonografia também foi realizada durante o cochilo. RESULTADOS: A latência de sono foi menor e a eficiência do sono maior no cochilo em L60 e L120 que em E60 e E120. O sono de ondas lentas nos cochilos em E120 e L120 foi mais longo que em E60 e L60. O tempo médio de reação em L60 ficou mais longo depois do cochilo e mais rápido em E60 e E120. Os cochilos em horário mais cedo serviram para neutralizar a queda no desempenho e funções fisiológicas durante o turno noturno. Houve uma ligeira melhora do desempenho ao se tirar um cochilo de duas horas num horário mais tarde durante o turno, mas houve piora depois de um cochilo de uma hora. CONCLUSÕES: Os cochilos na última metade do turno noturno foram melhores em termos de qualidade de sono do que os cochilos em um horário mais cedo. O desempenho, no entanto, piorou depois de um cochilo de uma hora em um horário mais tarde durante o turno por causa da inércia do sono. O estudo indica que o horário adequado de um cochilo curto deve ser cuidadosamente considerado, tal como um cochilo de 60 minutos durante o turno noturno