Effects of body position during an afternoon nap on body temperature and heart rate variability in healthy young Japanese adults

Objective : To examine the effect of body position during an afternoon nap on body temperature and heart rate variability in young healthy Japanese participants. Method : Within-subject laboratory experiment with two sessions. After sleep had been restricted the previous night, the participants were required to take a nap (60 min.) in a semi-recumbent position on a reclining chair at either 60 degrees (A) or 30 degrees (B) from the horizontal. The experiment was performed from 13 : 00 to 16 : 00 in the laboratory of Nursing Science in the School of Health Sciences, Faculty of Medicine, Kyoto University. An electrocardiogram (ECG), Polysomnography (Sleep electroencephalogram), core body temperature (rectal) and skin temperatures of the leg and foot were measured. Autonomic nervous function was evaluated by heart rate variability. Participants : Eight healthy Japanese men aged 19 to 24 yrs. Results : The decrease in rectaltemperature during the first 20 minutes was greater in B than in A. There were no significant differences in parasympathetic function between A and B, while the sympathetic function in B was more activated after the nap. In B sleepiness declined significantly after the nap. Conclusion : Napping in a posture similar to that when lying in bed deepened sleep adversely, with the possibility of a more prolonged phase delay.