Effects of two types of clothing offering different thermal insulation to the extremities upon nocturnal secretion of urinary 6-sulfatoxymelatonin and sleep propensity

Abstract

This study investigated skin temperature, the nocturnal secretion of urinary 6-sulfatoxymelatonin (aMT-6s), and self-estimated sleep propensity in subjects living in a thermally neutral environment but wearing two types of clothing which provided different thermal insulation to the extremities. The experiment was conducted from 21:30 h on day 0 to 06:30 h on day 2 in an environmental chamber controlled at 24 ± 0.2 °C and a relative humidity of 50 ± 5%. Seven healthy female volunteers participated twice, while wearing “covering” or “exposing” types of clothing. The “covering” type of experimental clothing consisted of long-sleeved shirts, full-length trousers and socks (Type L, weighing 1042 g); the “exposing” type of clothing consisted of half-sleeved shirts and knee-length trousers (Type H, weighing 747 g). During nocturnal sleep (from 22:30 h to 06:30 h), all subjects were covered by two thin sheets made of 100% cotton. When wearing Type H clothing, the nocturnal fall of forehead skin temperature tended to be greater (p = 0.07) and hand skin temperature was significantly lower during time awake (p < 0.05) with a greater nocturnal increase (p < 0.05). The distal-to-proximal skin temperature gradient, DPG, (a measure of the process of thermal regulation) was significantly lower with Type H clothing during times awake and asleep (p < 0.01 and p < 0.05, respectively), but the increase of DPG during the sleep period was greater with Type H clothing (p < 0.01). Type H clothing was also associated with a significantly greater secretion of nocturnal urinary aMT-6s during the first half of sleep (p < 0.05). Sleeping Inventory tests indicated that the subjects with Type H clothing had better subjective scores for sleepiness, sleep initiation, and sleep maintenance (all p < 0.05), and slightly less worries (0.05 < p < 0.10). These results suggest that clothing type uncovering the extremities in a thermally neutral environment of 24 °C might induce a greater day-night variation in heat loss and a larger nocturnal increase in skin temperature in the distal regions and promote more sleepiness and a more rapid sleep onset via increased urinary aMT-6s secretion in the first half of sleep period. It is suggested that these changes might be related to the role of the sympathetic nervous system in thermoregulation and melatonin secretion.

Keywords:

• clothing type
• heat loss from extremities
• urinary aMT-6s secretion
• distal skin temperature
• subjective sleep quality