Higher than average temperatures erode human sleep, the study shows

Most research examining the impact of climate change on human life focuses on how extreme weather events affect economic and public health outcomes on a large scale. However, climate change can also have a major impact on basic human activities – including many behavioral, psychological and physiological outcomes that are essential to well-being. In a study published May 20 in the journal One Earthresearchers report that rising ambient temperatures are affecting human sleep around the world.

The team says their findings show that by 2099, sub-optimal temperatures could undermine 50 to 58 hours of sleep per person per year. In addition, they found that the temperature effect on sleep loss was significantly greater for residents of lower-income countries, as well as for the elderly and women.

“Our results show that sleep – a major recovery process, an integral part of human health and productivity – can be worsened by higher temperatures,” said first author Kelton Minor (@keltonminor) of the University of Copenhagen. “In order to make informed decisions about climate policy, we need to take better account of the full range of plausible future climate impacts, extending from today’s society’s choice of greenhouse gas emissions.

Hot days have long been known to increase deaths and hospitalizations and impair human performance, but the biological and behavioral mechanisms underlying these effects are not well understood. Recent self-reported data from the United States suggest that subjective sleep quality decreases during periods of hot weather, but how temperature fluctuations can affect changes in objective sleep outcomes in people living in different global climates remains unclear.

“In this study, we provide the first evidence on a global scale that higher-than-average temperatures erode human sleep,” Minor said. “We show that this erosion occurs mainly by slowing down people’s sleep and progressing when they wake up during hot weather.

To conduct this study, researchers used anonymized global sleep data collected from accelerometer-based sleep tracking strips. The figures include 7 million nocturnal sleep records from more than 47,000 adults in 68 countries, covering all continents except Antarctica. The wrist strap measures used in this study have previously been shown to be consistent with independent wakefulness and sleep measures.

The study suggests that on very warm nights (more than 30 degrees Celsius or 86 degrees Fahrenheit), sleep decreases by an average of just over 14 minutes. The likelihood of getting less than seven hours of sleep also increases with rising temperatures.

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“Our bodies are highly adapted to maintain a stable body temperature, something our lives depend on,” says Minor. “However, every night they do something remarkable, without most of us consciously knowing – they emit heat from our heart into the environment, dilating our blood vessels and increasing blood flow to our arms and legs.” He adds that in order for our bodies to transmit heat, the environment must be cooler than us.

Early controlled studies in sleep laboratories found that both humans and animals slept worse when the room temperature was too hot or too cold. But this study was limited by how people act in the real world: they change the temperature of their sleeping environment to be more comfortable.

In the present study, researchers found that under normal living conditions, people seem to be much better able to adapt to lower outdoor temperatures than hotter conditions. “During the seasons, demographics and different climatic contexts, warmer outdoor temperatures constantly undermine sleep, with the amount of sleep loss progressively increasing as temperatures get hotter,” Minor said.

One important observation was that people in developing countries seem to be more affected by these changes. It is possible that the greater prevalence of air conditioning in developed countries plays a role, but researchers could not definitively identify the cause because they did not have data on access to air conditioning among the subjects. Researchers also note that because they have found strong evidence that the effects of warming temperatures on sleep loss are uneven worldwide, new research needs to look at particularly vulnerable populations, especially those living in the hottest and most historically the poorest regions in the world.

In its future work, the team would like to collaborate with global climate scientists, sleep researchers and technology providers to extend the scope of global analyzes of sleep and behavior to other populations and contexts. In addition, they are interested in studying the impact of rising outdoor temperatures on the sleep outcomes of inmates in hot climates who may have particularly limited access to air conditioning.

This study is supported by the Danish Agency for Higher Education and Science and the Independent Research Fund Denmark.

The story first appeared in News Medical

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