Climate change is coming for you in the night.
That’s the conclusion of scientists who study how heat disturbs sleep—and how projected warming is expected to make bad sleep even worse.
Led by Nick Obradovich of Harvard University’s Belfer Center for Science & International Affairs, a team of researchers are the first to document the relationship between rising temperatures and poor sleep. They sorted through how 765,000 Americans responded to Centers for Disease Control surveys from 2002 to 2011. Comparing the answers about sleep with respondents’ local temperatures, they not only found a link between temperature and sleep, but discovered that it was three times stronger in summer.
Their new study links that most individual of experiences—falling asleep—with a truly planetary phenomenon—global warming. It joins an expanding body of research suggesting that rising temperatures will gradually change even the most mundane aspects of everyday life.
The physiology of temperature and sleep has been understood for some time. As the body drifts toward unconsciousness, blood vessels in the skin dilate, which lets heat escape and brings down the core body temperature. Previous research has shown that above-normal ambient air temperature “can prevent core body heat shedding, and that poor sleep is associated with elevated core body temperature,” the authors write.
As the world warms, nighttime air temperatures increase at a faster rate than daytime mercury. This seemingly counterintuitive fact has been documented for years by scientists studying the global climate, making it a useful puzzle piece to keep in mind as America (unlike most of the world) struggles to accept climate change as a reality that’s caused by humans. (If the sun causes warming, wouldn’t you expect daytime temperatures to rise more?)
Having established a relationship between sleep and temperature, the researchers then used global warming projections to see how Americans’ sleep problems may worsen in the second half of the century.
Most surprising to Obradovich was that the researchers were able to see a strong effect of heat on sleep at all. The U.S. is a wealthy country, and many people can rely on air-conditioning during heat waves. “Even with this short-term adaptation capacity, we still observed effects in the U.S.,” he said.
When it comes to suffering through hotter temperatures, though, none are worse off than the elderly and the poor.
Solomon Hsiang, who studies the effects of climate change on human behavior and economics at the University of California, Berkeley, credits the new study as the first to methodically analyze temperature, climate, and sleep. The results, he said, “point toward systematic and important effects.”
Among them is an increase in the negative consequences that flow from being tired all the time. “People make cognitive errors that matter when they sleep badly, whether crashing vehicles or making poor decisions in the workplace,” Hsiang said. “Students learn poorly when they don’t sleep, and consistent lack of sleep harms people’s health. Sleep is so fundamental to all aspects of our lives that having the climate disrupt it many more days per month is a real and important cost that we now must consider seriously.”
The research team chose to run their sleep analysis through the scientific community’s worst-case climate scenario (which has come under some scientific scrutiny for including too much coal-burning, Bloomberg News reported Wednesday).
New research on greenhouse-gas trends and world coal reserves suggests two things about using this approach, known in the jargon as “RCP 8.5,” to project effects of climate change in the future (RCP stands for representative concentration pathways). First, emissions data show the world may be following a slightly less-cataclysmic, though still very bad, emissions scenario. (If hyperbole suits you, think of it as being diagnosed with stage 3 cancer instead of stage 4.) Second, even as an idealized worst-case scenario, RCP 8.5 may not be plausible as currently constructed.
Obradovich said that their study, published Friday in Science Advances, simulates the future as a linear extension of the sleep-survey and temperature data. If they’d used the next-worst-case climate scenario, which goes by the similarly lyrical moniker “RCP 6.0,” the team would probably have projected less disruption to sleep, he said. But while the exact pace of climate change remains a subject of debate, with questions about future fossil fuel consumption and feedback loops that may speed up warming, there is wide agreement on how this movie ends, and it’s not good.
Besides, the vagaries of climate modeling are secondary to the findings of the paper, Obradovich points out. “Unusually warm temperatures produce reports of worsened sleep outcomes, even in a wealthy, mostly temperate country like the U.S.,” he said. “The warmer the future world becomes, the more we anticipate sleep to be affected.”
The researchers’ next steps are to measure sleep quality more precisely, through sleep-lab studies and better monitoring of people’s activity.
It’s become great sport for climate deniers to belittle every new thing scientists say is affected by global warming, and sleep is sure to provide a doozy of a punch line. Less fun, though, will be the inexorable observation that if something as integral as sleep can be flummoxed by climate change, then why wouldn’t everything else be, too?