Even when it’s just a little warmer than usual, higher night temperatures mess with our slumber. A new worldwide study adds up these losses.
Everyone knows the horrible feeling: A stuffy night, just a little too warm, leads to restless sleep, and then next morning, you feel like a slow, groggy shell of yourself.
That feeling isn’t just unpleasant. Years of research show that sleep deprivation can ramp up heart disease risk, intensify mood disorders, slow one’s ability to learn, and much more—problems with big personal, societal, and economic costs.
Now a new study links sleep loss—and by extension, all the problems that come with it—with climate change. Researchers from the University of Copenhagen found that ever-warmer nighttime temperatures, nudged higher by climate change, push bedtimes later and wake times earlier, costing us precious nighttime rest.
Sleepers tracked in the study, published last week in the journal One Earth, lost rest even in places where temperatures weren’t blazingly high, and had trouble adapting to even mildly challenging sleep temperatures. And sleep costs, the researchers warn, will rise as temperatures do, potentially costing sleepers—that is, all of us—an extra 13 to 15 days of poor sleep each year by the end of the century.
It’s a very clear example of how climate change is playing out in people’s everyday lives, experts say—not just in catastrophic ways like more droughts and flooding, but in small costs that add up. Sleep loss from climate change “is already happening, right now, not in the future but today,” says Kelton Minor, the lead author of the study and a researcher at the University of Copenhagen.
A matter of degrees
Minor and his colleagues looked at data collected worldwide between 2015 and 2017 from nearly 50,000 people’s wristband activity trackers. The trackers recorded when those people fell asleep, woke up, and how they slept in between. Though the data were anonymized, the researchers could match sleepers’ locations with location-specific climate data.
That let the researchers compare the sleep data with local outdoor temperatures—they had no information about indoor conditions, or whether air conditioning was in use. Because they were looking at continuous records of individual people, they could see how someone slept on a cool night in June versus a hot one a few days later, or how they reacted to an unseasonably warm February night.
The dataset was unique in that it didn’t rely on self-reports, which are known to be unreliable. It also spanned the world, while the few previous studies looking at the direct relationship between climate and sleep focused on just a few people, or just the United States.
Bodies don’t adjust
Possibly more concerning, though, was another finding: People’s bodies didn’t seem to adjust to warmer sleeping temperatures—even if they lived in hot climates year-round, or even after they’d lived through a summer of hot-night exposure. Warmer-than-usual nights messed with their sleep no matter what.
That makes sense, he explains, given how tightly our bodies regulate their internal temperature. A few degrees too hot or cold and our organs start to function less well or shut down. Body temperature is one of the primary controls on sleep: Before bedtime, we shunt blood toward our extremities and cool our cores ever so slightly. Without that shift, sleep gets much harder to slip into.
Needing such tight control over our body temperature makes us less flexible in the face of worsening sleep conditions.
Already, human-caused climate change has warmed the planet by about 2°F (1.1°C) since the 1800s. But nights have warmed more than days in most parts of the world; in the U.S., summer nights have warmed twice as much as summer days.
The Copenhagen researchers estimated that warmer nights already cost sleepers about 44 hours of rest every year. There are also 11 extra days of “short sleep,” nights when sleepers get less than seven hours.
But as the planet heats further, those costs will increase. By the end of the century, sleepers could lose 50 hours per year if carbon emissions continue more or less on their current track.
A problematic solution
This is not a thing to take lightly, says Jose Guillermo Cedeno Laurent, an environmental health researcher at Harvard. He and his colleagues did an experiment at Harvard College during a 2016 heatwave. Students who slept in newer dorms with air conditioning did better on cognition tests in the following days than those living in older buildings “built for another climate,” says Cedeno Laurent.
His study points to one possible solution to climate-induced sleep deficits: getting lots more air conditioning in households worldwide. But that’s a huge economic and environmental challenge. AC costs a lot of energy and therefore money; a recent study shows that U.S. low-income households wait until temperatures are 5-7.5°F hotter before they turn on their cooling systems. And at the moment, AC actually warms the outdoor environment both globally, because most of the electricity for it comes from burning fossil fuels, and locally, because the excess heat sucked out of bedrooms gets dumped into the air outside.
