Decline in the quality of sleep increases the morbidity and risk of lifestyle diseases. In addition, daytime sleepiness causes traffic accidents and decreased work efficiency. According to a survey conducted by the Japanese Ministry of Health, Labour and Welfare, 20% of people experience sleep problems, and decline in sleep quality in particular is serious. One factor that has been linked to decline in sleep quality is high heat and humidity in summer.
Against this background, some studies have examined the effects of thermal environment control on sleep. Appropriate use of air conditioning has been shown to improve sleep quality; however, maintaining a good indoor thermal environment using air conditioning increases energy consumption. Given the present state of climate change, the reduction of energy consumption in the Japanese residential sector is a pressing issue. However, an excessive reduction in air conditioning use for the purpose of saving energy could create a poor sleep environment. The effects of thermal environment control using air conditioning on sleep quality and energy consumption have been independently investigated in several previous studies.
In this research, we elucidate the impact of thermal environment control on both sleep and energy consumption during the summer. The effect of thermal environment control on sleep and energy saving was unified as a monetary value. Increase in energy consumption can be estimated as an increase in air conditioning costs. Conversely, because reduced sleep efficiency translates to reduced working efficiency, improved sleep efficiency was converted into monetary value as a reduction in wages.
Eight healthy male university students participated in this study. The study was performed in a two-story maisonette room in an apartment building with high environmental performance. Experiments were conducted in August 2013 and 2014. In the 2013 experiments, we examined the effect of thermal environment control on sleep and energy consumption. Three thermal environmental control scenarios were used: an air conditioner set at continuous operation at 26.0 °C (Case I); an air conditioner set at continuous operation at 28.0 °C (Case II); and natural draft only (Case III). The increase in air conditioner temperature setting from 26.0 to 28.0 °C used in this study resulted in a 0.03 kWh/day/m2 reduction in 8-h energy consumption (Fig. 6). Sleep efficiency was significantly higher for Case II (average SET* during sleep, 27.4 °C) (Fig. 8). Low SET* caused by excessive air conditioner use and high SET* caused by not using an air conditioner adversely affected sleep efficiency. The relationship between sleep efficiency and work efficiency was verified in the experiments conducted in August 2014, confirming that the economic impact of decreased sleep efficiency could be converted to a monetary value. Working efficiency decreased by 1.1% per 1% decrease in sleep efficiency (p<0.01) (Fig. 13). The combined economic impact of energy saving and sleep quality was calculated. The economic impact of improved sleep quality was much greater than that of energy saving, and the economic value of Case II (average SET* during sleep, 27.4 °C) was the highest (Fig. 14).
This study found that sleep quality influences work efficiency, demonstrating the importance of improving sleep. To create a comfortable sleeping environment, further verification of thermal environment control in more detail is necessary.