In this study, field surveys in actual bedrooms in summer and winter were conducted with the purpose of verifying that the evaluation of the thermal environment in bedrooms by calculating the human heat load on the human body is effective in different seasons. The measurements were undertaken in two seasons; the summer period was July, August and September of 2017 and 2018, the winter period was December of 2017, and January and February of 2018. In the calculation of the human heat load, the actual basal metabolic rate of the subjects was substituted for the metabolic rate in formula proposed by Akiyama et al. during sleep. The quality of sleep was evaluated based on the physiological parameter during the sleeping period, including sleep stage, by measuring the electroencephalogram, electrooculogram, and electromyogram and the subjective sleeping feeling at the time of waking up. We evaluated that sleep quality was good as there were more appearances of deep sleep and less appearances of wake time. Also, OSA sleep inventory was used for subjective evaluation of the sleeping feeling.
Since the relation between human heat load and percentage of sleep stages in winter was similar to that in summer, it was shown that the evaluation of the thermal environment in bedrooms due to human heat load may be effective even in different seasons. In both seasons, the colder environment when the human body thermal load is smaller than 0 W / m2 decreased the quality of sleeping, but it tended to be different in warm environments when the human heat load was greater than 0 W / m2 ; especially around 45 minutes after turning over, there was a tendency that the percentages of stage wake, REM sleep, and shallow sleep are the smallest and the percentage of deeper sleep is the largest in the vicinity of the steady state where the heat loss to the bedding from the human body is nearly 0 W / m2.
The value after sleep onset immediately was taken as the initial value, and the heat radiation amount or endotherm amount added every 30 seconds was calculated as ΣLm. The closer the ΣLm is to 0 kJ/m2 the higher the percentage of deep sleep. That is, the quality of sleep tended to be improved in the state where the heat radiation amount and the endotherm amount were about the same.
It is known that deep sleep appears longer during the initial part of 90 minutes after sleep onset, and the sleep becomes shallow in the latter part of sleep. In this study, the deep sleep time overnight tended to be longer when a longer deep sleep time was obtained within 90 minutes after sleep onset. In addition, the longer the deep sleeping time in one night, the more the subjective feeling of sleep when waking up tended to be improved. Therefore, in this study, it was shown that, in order to improve the quality of sleep, it is important to keep the body posture in the bed long in a low-humidity and warm environment because the heat flow to the bedding from the human body continues in the steady state quantitatively. Finally, it was shown that it is important to get a longer deep sleeping time from both viewpoints of physiological evaluation and psychological evaluation for sleep.
