Japan Cities Lose 34 to 50 Hours of Sleep Yearly From Hot Nights, With 9-14% Tied to Climate Change

The rainy season ended in many parts of Japan over the past few weeks, and summer arrived with a sudden surge in both daytime and nighttime temperatures, The Japan Times reported. Urban homes in concrete-heavy cities retain heat from asphalt and buildings, turning rooms into persistent saunas that disrupt sleep without heavy air conditioning use.

Climate Central released an analysis last week covering 1,338 cities worldwide.

The group found that temperature-related sleep loss linked to climate change had at least doubled since the early 1970s. Over the five years through 2025, the average person globally lost nearly 56 hours of sleep per year due to high temperatures, with more than 10 percent of that loss attributed to climate change from burning fossil fuels and deforestation.

The study drew on a 2022 model by scientist Kelton Minor that tracked sleep data from over 47,000 people across 68 countries wearing fitness wristbands and matched it to local weather records.

Once outdoor nighttime temperatures exceed 10 degrees Celsius, sleep duration declines sharply. Above 25 C, the share of people sleeping less than seven hours rises by 3.5 percent. In Japan the analysis covered 36 municipalities with populations of half a million or more.

Kristina Dahl, Climate Central’s vice president for science, stated that the analysis reveals how climate change translates into measurable hours of lost sleep. She added that impacts of fossil fuel-driven warming extend beyond extreme weather to undermine one of the most fundamental requirements for human health.

Tomohiko Ihara, associate professor at the University of Tokyo, reached similar conclusions in a 2022 study of 1,284 people in Nagoya. Health began to worsen when outside air temperatures rose above 24.8 C, and 40 percent of respondents reported sleep problems once the lowest daily temperature reached 30 C.