A pilot study of the effect of bathing time on thermal sensation to get a good night's sleep
Authors: Mitsuno Tamaki , Moriya Sayaka 
Published: June 2023

In Japan, cold sensitivity is an indefinite complaint that affects many women. However, since it is not a life-threatening serious condition, it is difficult to treat or research the target. However, women who are aware of sensitivity to cold feel the cold most when they go to bed, and there are many people who suffer from the coldness that it is difficult to fall asleep even in a heated environment in winter. In this study, we investigated the effects of bathing, which is likely to raise body temperature, core temperature, skin temperature, and thermal sensation, and prevented coldness. The method of getting a good night's sleep was proposed without a cold sensation.The subject was a 21-year-old healthy woman who has heavy sensitivity to coldness. Experiment I: Room temperature and core temperature (oral temperature) were measured every 60 minutes from 7:00 to 23:00. Experiment II: After taking bath at 20:00, the room temperature, her core temperature, skin temperature of instep/toe, and the thermal sensation of the feet were measured every 20 minutes from 21:00 to 23:00. Experiment III: Two bath times were set from 20:00 or 22:00. The skin temperature of the instep/toe, core temperature, and thermal sensation were measured before and after bath time. Experiment IV: When she felt a cold sensation, core temperature, skin temperature of the instep/toe, and thermal sensation were measured before and after taking the 10 minutes foot bath at 23:00.<Results and Discussion> Result I: From the correlation coefficient between body temperature and room temperature, which had a significant positive correlation, indicating that the higher the room temperature, the higher the body temperature. Room temperature increased significantly from morning to noon and decreased significantly in the evening. However, body temperature increased temporarily from evening to night and decreased significantly in the middle of the night. Result II: The oral temperature was significantly higher than before bathing until 21:20, and then significantly decreased from 22:00. Skin temperature of the instep was significantly higher until 22:00 and one of the toes was significantly higher until 21:40 than before bathing. The thermal sensation was significantly higher until 21:40 compared to before bathing and was evaluated as ``warm''. We calculated the relationship between the skin temperature of the instep/toe and the coldness sensation. The skin temperature (X1: instep, X2: toe) was explained by the equations: Y1 = 0.20X1-5.92 (R2 = 0.518) and Y2 = 0.14X2-3.67 (R2 = 0.667). The skin temperature of the instep/toe (X1/X2) was comfortable thermal sensations, Y1 = 0 (instep) and Y2 = 0 (toe), these skin temperatures were 29.6°C/26.2°C. Result III: Both oral and skin temperatures and their thermal sensations were increased significantly after bathing at 20:00 or 22:00. Although the increasing rate of core temperature at 20:00 bath time was significantly higher than at 22:00, however, there was no difference in the skin temperature of the insteps/toes and the thermal sensation. Result IV: Oral temperature increased after 22:00 bathing (Experiment III), but not after foot bathing. However, both thermal sensations of taking bath or footbath increased. The rate of skin temperature of toes in the footbath was increased significantly higher than taking bath and was maintained until bedtime. Therefore, taking a footbath at 23:00 would make it possible to sleep comfortably.