Hot summer nights have bad impact on health: Study

A survey of Nagoya inhabitants in central Japan found evidence of the harmful impact of higher evening summer temperatures on population health.

Over the summers of 2011 and 2012, more than 1,200 people completed a sleep quality questionnaire. The results revealed that when the daytime temperature climbed above 24.8 degrees Celsius, sleep disruption increased at night. The research was published in the journal Sleep and Biological Rhythms. The number of population health years lost was equivalent to the consequence of heatstroke when using the disability-adjusted life year (DALY) assessment. This emphasises the significance of not just managing rising daytime temperatures, but also of finding methods to attain appropriate overnight temperatures so people may sleep properly.

Hot summer nights might be nice when you’re on holiday, but in Japan, things have been getting a bit too warm for comfort. Talk to a neighbour and the most common greeting will be, “It’s hot, isn’t it!” It’s uncomfortable, to say the least, but what other impacts are these hotter and longer summers having on Japan’s sweltering residents?

Researchers from the University of Tokyo conducted a survey with residents from Nagoya to find out how rising summer temperatures might be affecting their sleep. Located in the centre of Japan, the city is an urban heat island which has experienced the largest temperature rise among Japan’s three major metropolitan areas (comprising also Tokyo and Osaka).

In total, 574 adults in 2011 and 710 adults in 2012, representative of the age and sex ratio of Nagoya’s population, completed an online survey over about 10 days each year.

“The survey, called SQIDS2 (sleep quality index for daily sleep) was a revised and simplified version of a widely used, standardized sleep survey called PSQI (the Pittsburgh Sleep Quality Index).

It assessed seven components, including subjective sleep quality, how quickly people fell asleep and use of sleep medication,” explained Associate Professor Tomohiko Ihara from the Graduate School of Frontier Sciences. “Unlike with PSQI, the new SQIDS2 survey allowed us to correlate overall sleep quality with daily temperature.”

The team found that the daily prevalence of sleep disturbance increased when the daytime minimum temperature went above 24.8 degrees Celsius. There was also a significant difference in sleep disturbance between age groups for the male participants, with younger men having more trouble getting a good night’s sleep than older men.

Using the results from SQIDS2, the team calculated the DALY — disability-adjusted life year — for the city’s more than 2.2 million residents. DALY is a measure of time lost through premature death and time lived in states of less-than-optimal health at a population level (rather than for individuals).

Used by the World Health Organization (WHO), DALY helps to quantify loss from hundreds of different diseases, injuries and risk factors, which can then be used to help improve health care systems.

“We calculated that the DALY score for heat-related sleep disturbance was 81.8 years in 2012, which is similar to the score for heatstroke. That means that the damage to health caused by sleep disorders due to rising temperatures is comparable to that of heatstroke and must be addressed,” said Ihara. “Sleep disorders increase when the minimum daily temperature exceeds 25 degrees Celsius.”

There were several limitations to the study, not only due to the target population size and restricted age range (children were not involved and the number of respondents over 70 years was relatively small) but also because other influences on sleep quality were not investigated, such as the mental health of the participants or whether they used an air conditioner.

However, this study is a step towards quantifying the damage caused by climate change to sleep health, and the researchers hope it will help climate change legislators recognize the significant impact of high nighttime temperatures and be used to provide guidance for better sleep.

“Air conditioners are widely installed in Japan and so their appropriate use may be one solution. I try to acclimate my body to the heat as much as possible, but when it is unbearable, I turn on the air conditioner both day and night,” said Ihara. “But this option is not available to everyone and in the long run, promoting measures to reduce outdoor temperatures, both during the day and at night, will be needed.”

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