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.
Takeyama
Hidemaro
H
Health Sciences of Life, Work and Environment, Department of Environmental Health Science and Health Promotion, Nagoya City University Graduate School of Medical Sciences, Nagoya, Japan. hidemaro@med.nagoya-cu.ac.jp
Matsumoto
Shun
S
Murata
Kensaburo
K
Ebara
Takeshi
T
Kubo
Tomohide
T
Tachi
Norihide
N
Itani
Toru
T
D000328:Adult; D000704:Analysis of Variance; D002940:Circadian Rhythm / Q000502:physiology*; D004568:Electrodiagnosis; D006801:Humans; D008297:Male; D011930:Reaction Time; D012146:Rest / Q000502:physiology*; D012892:Sleep Deprivation / Q000503:physiopathology*; D011795:Surveys and Questionnaires; D013647:Task Performance and Analysis; D013997:Time Factors; D014940:Work Schedule Tolerance / Q000502:physiology*