Effects of softness of bedding materials upon overnight excretion of urinary catecholamines and sleep quality in warm environmental conditions

The purpose of this study was to investigate whether, when subjects were living in a moderately warm environment: (1) the softness of clothing worn during the daytime could influence the subjects’ preference in the evening for the softness of clothing and a face towel; and (2) the softness of bedding materials could modulate their nocturnal body temperature, overnight urinary catecholamine excretion, and sleep quality. Six females were tested during the luteal phase of their menstrual cycles. The experiment was conducted over three consecutive days and nights in a climatic chamber controlled at 28 ± 0.2 °C and 50% RH during the evening (from 19:30 to 21:30 h) and at 29 ± 0.2 °C and 50% RH during the sleep period (from 22:30 to 07:00 h). The first night was for adaptation to the experimental chamber. Five different sets of clothing and bedding were used; these were identical except for the softness/hardness of the materials used (due to treatment with fabric softener or starch), and material softness decreased in the order: Type A (softest)?>?Type B > Type C > Type D > Type E (hardest). There were two phases to the experiment, conducted in random order. In one phase, subjects wore “soft type” (Type B) T-shirt and shorts in the daytime and, in the other phase, “hard type” (Type D/E) T-shirt and shorts. In both phases, subjects were asked at 21:30 h to select a T-shirt and a face towel which they felt would be most comfortable to use. At night, they slept on bedding (a mattress cover, a pillowcase and a covering blanket) which was of the same degree of softness as the T-shirt and shorts that had been worn in the daytime in that phase. Rectal temperature, skin temperatures at seven sites, and body movement were measured during sleep, an overnight urine sample was taken for measurement of urinary excretion of adrenaline and noradrenaline, and subjective sleep quality was assessed on being woken the following morning. The main results were as follows: (1) T-shirt preference in the evening showed large inter-individual variation but did not differ significantly between the two phases (when “hard type” or “soft type” clothing had been worn during the daytime). The preferred texture of the face towel was softer than that of the T-shirt, the difference in softness between the chosen face towel and T-shirt being significant (p < 0.05 and p < 0.10, respectively, when having worn “soft type” and “hard type” clothing in the daytime). (2) Rectal temperature and mean skin temperature were not significantly different when sleeping with “soft type” (Type B) and “hard type” (Type D/E) bedding materials. (3) Overnight secretions of urinary adrenaline and noradrenaline were significantly less with “soft type” than “hard type” (p < 0.01 and p < 0.05 for adrenaline and noradrenaline, respectively) bedding. (4) When sleeping with “soft type” bedding materials, five out of the six subjects showed less body movements during the sleep period and reported that they had slept better. These results suggest that, in a moderately warm environment, bedding materials with a softer texture might be more comfortable to the subject (due to less tactile stimulation of the skin, which results in neurophysiological relaxation) and provide them with better sleep quality.