But global warming is already affecting an everyday and vital action for health: sleep.
A new study published in the journal One Earth by researchers in Denmark and Germany points out that rising temperatures are negatively affecting night’s rest globally.
“Our study presents the first evidence on a planetary scale that warmer-than-average temperatures are eroding human sleep,” said Kelton Minor, a researcher at the University of Copenhagen and lead author of the study.
“We show that this erosion occurs primarily when the time people fall asleep is delayed and the time they wake up during hot weather progresses.”
How much sleep is being lost
The researchers used sleep monitoring wristband data from 47,000 adults between 2015 and 2017 in 68 countries.
The researchers claim that by the end of the century suboptimal temperatures could cause us to lose on average between 50 to 58 hours of sleep a year.
But Minor emphasizes that the impact of climate change on rest is already a reality.
Lack of sleep affects health and cognitive abilities.
People are already losing an average of 44 hours of sleep each year.
The scientists also found that people sleep more when the outside temperature is below 10 °C.
Above that threshold increases the likelihood of sleeping less than seven hours.
And when the outside temperature exceeds 30 °C, people lose on average about 15 minutes per night.
Although it may seem like a minor loss, this decrease could take place in the stage of deep, restful sleep called slow-wave sleep, the first phase of which lasts about an hour.
Which are the most affected groups
The impact of temperature on sleep increases with age.
People over the age of 70 lose about 30 minutes instead of 15 with a similar temperature.
Women are also more affected.
The study notes that “under identical conditions, women’s core body temperature decreases earlier in the evening compared to men, possibly exposing women to higher ambient temperatures around their usual sleep onset time.”
“Women have also been shown to have a thicker subcutaneous fat, which could affect nighttime heat loss.”
On the other hand, people in low- and middle-income countries suffer an impact on sleep about three times greater than those in high-income countries.
It’s possible that the higher prevalence of air conditioning in developed countries plays a key role, but the researchers clarified that they couldn’t quantify its importance because they didn’t have data on access to air conditioning among study participants.
The impact of temperature on sleep increases with age.
Lack of sleep not only affects health but also our cognitive performance, according to previous research.
A study by José Guillermo Cedeño Laurent and colleagues at Harvard University measured the impact of a 2016 heat wave on students.
Students who slept in newer, air-conditioned dormitories performed better on cognitive tests in the following days than those who lived in older buildings less adapted to rising temperatures.
Body temperature
Hot days have long been known to increase deaths and hospitalizations, but the biological mechanisms underlying these impacts are not yet well understood.
“Our bodies are highly adapted to maintain a stable core body temperature, something our lives depend on,” Says Minor.
“However, every night they do something remarkable without most of us knowing it consciously: they release heat from our center into the environment by dilating our blood vessels and increasing blood flow to our hands and feet.”
Minor adds that for our bodies to transfer heat, the surrounding environment must be cooler than we are.
The researchers found that people have not changed their daily behavior to cope with lack of sleep, such as taking a nap.
The scientists found no evidence that it is easier to sleep on a warm night in late summer than on a warm night in early summer. “We found no evidence that people are adapting well,” Minor said.
“The full spectrum of climatic effects”
Global temperature at the earth’s surface level was on average 1.09°C higher between 2011 and 2020 than between 1850 and 1900, according to the August 2021 report of the UN Panel on Climate Change, IPCC.
A report by the Climate Action Tracker analysis center points out that, without drastic cuts and taking into account current policies, the world is on track for a temperature increase by the end of the century of 2.7 °C compared to pre-industrial levels.
People in low- and middle-income countries suffer three times the impact on sleep than those in high-income countries.
The authors of the new study believe that due to the uneven impact on sleep in different populations, future research should especially consider the most vulnerable populations, particularly those residing in the hottest and poorest regions of the world.
Minor notes that the impact of temperature on sleep should be included in more studies on the possible consequences of global warming.
“To make informed decisions about climate policies in the future, we need to take more into account the full spectrum of plausible future climate impacts,” he said.
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