TOKYO -- Some 40 percent of people have difficulty sleeping when nighttime temperatures stay above about 24 degrees Celsius during the summer months, researchers have found.
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Global warming and heat islands -- urban areas hotter than their rural surroundings -- have increased the number of days when it stays hot even at night, impacting sleep quality, the research team led by University of Tokyo associate professor Tomohiro Ihara said. The team is calling on people to use air conditioning at night when appropriate, as reduced sleep quality not only affects health but also reduces efficiency at work.
"There is a lot of attention paid to heatstroke that can lead directly to death, but low sleep quality has a far larger overall impact on society," commented Ihara.
While many people have trouble sleeping on summer nights, the connection between a good sleep and the temperature was not well understood. To quantify sleep quality, Ihara's team revised a questionnaire commonly used overseas.
The researchers assigned numerical values to answers to questions including, "Do you wake up during the night?" and, "Do you ever find it impossible to stay awake during normal daily activities such as driving or eating?" They then judged that totals over a certain score indicated the subject had difficulty sleeping.
The questionnaire was completed by 1,284 people in Nagoya, in August 2011 and July and August 2012. Results from both periods showed about 40 percent of respondents on average had trouble sleeping. The average outdoor temperature at midnight during the survey periods was between 23 and 31 degrees Celsius, and the survey found that the number of subjects having trouble sleeping spiked whenever the midnight temperature exceeded 24.3 degrees.
(Japanese original by Ryo Watanabe, Osaka Science & Environment News Department, and Ai Oba, Tsukuba Bureau)