Everyone knows the frustration of bad sleep on a hot night, tossing and turning in the sheets as the hours slip away.
That experience is becoming more common thanks to climate change, according to a new study from the research organization Climate Central. The analysis finds that on average, people around the world lose about 56 hours, or seven full nights of sleep each year to uncomfortably warm temperatures. About 10% of that lost sleep—roughly 5 hours—can be pinned to human-driven climate change.
"Summertime is hot. It's always been," says Kristina Dahl, the lead scientist at Climate Central. "But climate change is making them hotter, and making nights hotter. So it's really amplifying the sleep loss that would happen naturally."
The study looked at temperature records from 1,300 cities around the world. The Climate Central team knew from a study published a few years earlier that the amount of time people sleep is tightly tied to overnight outdoor temperatures. So they could extrapolate the total time people spent sleeping based on the temperatures where they lived. That number represented the real sleep people got in a world that has been heated up by climate change.
Then, the Climate Central researchers used a climate model that presents a hypothetical world: one where human-driven climate change hasn't happened. The researchers could estimate temperatures in all those same locations—and see how much people would have slept in that cooler scenario.
The difference, Dahl says, isn't enormous for each person: it's on the order of 5 hours a year so far. But that number has doubled since the 1970s, and it will continue to rise as climate change marches onward, she says.
That's in part because overnight temperatures have crept upward much faster than daytime ones. The causes are complex, but one factor is the shape of the atmosphere: At night, the layer of air closest to the ground, where people actually live, is short and compact. During the daytime, that layer expands. Heat absorbed during the day gets trapped into that compact band of air during the night, pushing temperatures up.
Sleep is tightly tied to temperature. Sleep laboratory studies consistently show that hotter and more humid conditions degrade sleep quality. People's ability to fall asleep in the first place decreases, and the sleep that does occur gets fragmented and less restorative.
Particularly, says sleep doctor Daniel Rifkin from the sleep consultancy group Ognomy Sleep, the type of sleep that leads to dreams: REM.
"If it's too hot," he says, "your REM sleep is extremely broken, and we lose our dreams."
Sleep lab studies helped scientists understand the best temperature conditions for sleeping indoors. But until recently, it wasn't clear how real-world weather conditions affected people's sleep.
But a few years ago, environmental data scientist Kelton Minor and his colleagues set out to figure out that relationship. They collected sleep data from nearly 50,000 Fitbits around the world, along with detailed overnight temperature data from sleepers' locations.
"The warmer it gets, the less that people sleep," says Minor— especially once nighttime temperatures crept above about 50 to 59 degrees Fahrenheit. Beyond that, Minor and his colleagues found, "every additional increase per degree on that hot side leads to progressively less sleep."
The new Climate Central analysis used Minor's sleep-temperature relationship to estimate climate-induced sleep loss.
Sleep is already a precious and threatened commodity, says Minor. In 2015, the Centers for Disease Control and Prevention declared sleep problems like disorders and simple insufficiency to be a public health crisis.
So far, the climate impact adds up to just a few minutes a day during summertime. But because the relationship between sleep loss and health is so strong, the cumulative impacts of that loss on public health could be significant, says sleep doctor Rifkin. The impacts also fall hard on people who don't have access to cooling technologies such as air conditioning, or who can't afford to run them.
"Heat really finds the vulnerable people first," he says. "It affects those the most that need the help the most."
He recommends doing whatever possible to cool your sleep space. If nothing else is possible, sticking your feet outside the sheet or blanket can help your body shed heat and cool down enough to sink into sleep.
Beyond those recommendations, Dahl says one of the best sleep aids isn't a pill or a meditation app, but cutting back on burning fossil fuels, the primary driver of climate change.
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