Scientific observations over the years have shown that nights are getting warmer faster than days (in fact, cooler regions and cooler seasons and times of day are warming up faster than their hotter counterparts). So given that human beings spend a third of their lives sleeping, what kind of an impact do warmer-than-usual nighttime temperatures have on sleep quality? That's the crux of the question behind a real-world study to establish how unusually warm nights might be linked to sleep disruption, as well as how projected global warming could make things worse.

How temperature affects your sleep

As the body prepares for sleep, the blood vessels in the skin dilate to cool down the core body temperature by letting heat escape. As we sleep, it remains low throughout the night and rises again shortly before we wake up. Above-average ambient temperatures prevent core body heat from escaping, which in turn contributes to poor sleep, note the authors in the study.

"Sleep has been well-established by other researchers as a critical component of human health," says lead author and political scientist Nick Obradovich, who was inspired to investigate the effect of climate change on sleep back in 2015 when he was still a PhD student at the University of California San Diego. The city had been in the throes of a heat wave that had lasted all the way into the fall and he had noticed the toll it had taken on him as well as other students. While numerous other studies have predicted how climate change could wipe out life in the sea and on land, cause more volcanic activity and cause carbon dioxide levels to hit the roof, no one has looked at how it could affect one of the most essential human activities. "Too little sleep can make a person more susceptible to disease and chronic illness, and it can harm psychological well-being and cognitive functioning. What our study shows is not only that ambient temperature can play a role in disrupting sleep but also that climate change might make the situation worse by driving up rates of sleep loss."

Sifting through 765,000 responses from a public health survey conducted by the Centers for Disease Control and Prevention between 2002 and 2011, the study shows a link between reports of insufficient sleep and higher nighttime temperatures. The main takeaway from the study is that nightly temperatures averaging 1 degree Celsius higher than normal is enough to result in three nights of insufficient sleep per 100 individuals per month. To put things in perspective, this translates to nearly 9 million additional nights of insufficient sleep per month or around 110 million extra nights of tossing and turning annually when applied to the current US population as a whole.

Scientists predict the western and northern areas of the United States will likely be affected most by rising global temperatures(Credit: N. Obradovich)

The future doesn't look great either: based on worst-case scenario climate projections by NASA Earth Exchange, if current global temperatures are allowed to climb at a rapid pace, this number could double to six extra nights of lying awake per 100 individuals per month by 2050 and approximately 14 extra nights per 100 by 2099. For those who have the means to let the air-conditioner run throughout the night, this won't be too much of a concern. Unsurprisingly, the people who will be hardest hit by this are likely to be the elderly, who have trouble regulating their body temperatures, and those in the lower-income bracket who earn $50,000 or less.

That said, this study isn't without its limitations. While actual sleep loss due to higher-than-usual nighttime temperatures is a factor, there are many other reasons why people experience insufficient sleep. Whether or not people do manage to get sufficient sleep in a hotter climate depends on factors such as the use of technology, hours worked, stress levels, and changes to the built environment, note the authors.

Those living in wealthy countries, such as the US and those in Western Europe, will no doubt find a way to adapt to rising global temperatures and all the issues (which are potentially much worse than sleep deprivation) that entails, but what about developing countries in regions such as Southeast Asia and Sub-Saharan Africa where people live on less than US$3.10 a day? According to a 2014 report released by the Intergovernmental Panel on Climate Change, the spike in global temperatures is likely to have the hardest impact on people living in coastal regions in Asia. Among the projected climate trends, more frequent and intense heat waves will not only increase the rate of disease and death but also lead to a decline in productivity due to heat stress. One way or another, people, especially those in vulnerable populations, will be losing sleep over global warming.

"The U.S. is relatively temperate and, in global terms, quite prosperous," says Obradovich. "We don't have sleep data from around the world, but assuming the pattern is similar, one can imagine that in places that are warmer or poorer or both, what we'd find could be even worse."

The study was published in PLOS One.

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