In Japan, the total financial loss caused by sleep disorders is estimated to be about 3.5 trillion yen annually. Chronic sleep disorders are a factor in lifestyle-related diseases and can affect life expectancy. The Japanese government has set guidelines for improving sleep-related problems (e.g., Healthy Japan 21). However, the goal for improvement of sleep-related problems, —a reduction from 18.4% in 2009 to 15.0% in 2022 in the percentage of people who do not receive adequate rest from sleep—remains unachieved, and the percentage has actually increased (20.0% in 2014).
Thus, the effect of indoor environment on sleep has attracted attention, as it greatly affects sleep quality, especially in summer and winter. However, studies of the effects of the indoor thermal environment are relatively scarce in winter compared with summer. Additionally, most winter studies are field surveys of daily life, so it is difficult to clarify the exact relation between indoor temperature and sleep quality while accounting for differences between individuals. This study therefore aimed to clarify the impact of indoor thermal environment on sleep quality by focusing on changes within individuals before and after moving to houses with high thermal insulation performance.
Follow-up investigations on sleep quality and indoor temperature were conducted in winter (2014-2016) before and after moving. Sleep quality was measured by subjects with non-contact sleep-monitoring equipment, which observes sleep/waking and sleep depth by detecting body movement using an electric wave sensor. Indoor temperature at 1.1 m above the floor was measured in the living room, bedroom, bathroom, and dressing room at 10-min intervals. During the survey periods, subjects recorded their waking time, bedtime, and whether they consumed alcohol in a daily diary. Questionnaire surveys on personal characteristics and housing were conducted. Questionnaire surveys on housing were also distributed to building contractors to receive reliable answers.
Thermal insulation performance of houses before moving were found to be at “pre-1980 standards” or “1980 standards” levels, whereas that of houses after moving satisfied “1999 standards”. There was little change in total Pittsburgh Sleep Quality Index score, but the frequencies of feeling cold in bed and difficulty falling asleep and were significantly decreased after moving, as were amount of clothing worn to bed and number of comforters used. Average daily outdoor temperature before moving (6.2 °C) was nearly equal to that after moving (6.4 °C). However, bedroom temperature when going to bed was higher after moving (17.1 °C) than before moving (15.8 °C), and the bedroom temperature when getting out of bed was significantly higher after moving (15.9 °C) than before moving (13.8 °C).
In the surveys before moving, measurement data were consistent with questionnaire responses. Sleep latency was significantly longer in subjects who reported difficulty falling sleep than in others. The observed number of nighttime awakenings was significantly higher in subjects reported nighttime awakenings than in others. Subjects who reported feeling cold in bed had lower sleep efficiency and more nighttime awakenings compared with subjects who did not feel cold. Multiple linear regression analysis found that sleep efficiency increased by 0.73% and number of awakenings decreased by 0.18 occurrences per 1 °C increase in indoor temperature during sleep. The results indicate that to obtain high-quality sleep, simple behavioral thermoregulation such as adjusting clothing or bedclothing is insufficient and that it is important to maintain an appropriate indoor temperature during sleep.
