Global warming is already reducing sleep time. And the effects are worse in low- and middle-income countries Many studies on the impacts of climate change focus on extreme events such as hurricanes or floods. But global warming is already affecting a daily and vital action for health: sleep.
A new study published in the journal One Earth by researchers in Denmark and Germany finds that rising temperatures are negatively affecting nighttime sleep globally.
“Our study presents the first planetary-scale evidence 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 mainly when the time people go to sleep is delayed and the time they wake up is advanced during hot weather.”
How much sleep are you missing?
The researchers used data from sleep-monitoring wristbands from 47,000 adults between 2015 and 2017 in 68 countries. Researchers say suboptimal temperatures could cause us to lose an average of 50 to 58 hours of sleep a year by the end of the century.
But Minor emphasizes that the impact of climate change on rest is already a reality. 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, the probability of sleeping less than seven hours increases. And when the outside temperature exceeds 30°C, people lose an average of 15 minutes a night.
Although it may seem like a minor loss, this decrease could occur in the deep, restorative stage of sleep called slow-wave sleep, the first stage of which lasts about an hour.
Which groups are most affected?
The impact of temperature on sleep increases with age. People over 70 lose about 30 minutes instead of 15 at 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 time of sleep onset.” “It has also been shown that women have a greater thickness of subcutaneous fat, which could affect nocturnal heat loss.”
On the other hand, people in low- and middle-income countries experience around three times more sleep impact than those in high-income countries. The higher prevalence of air conditioning in developed countries may play a key role, but the researchers said they were unable to quantify its importance because they did not have data on access to air conditioning among study participants.
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 dorms performed better on cognitive tests in the following days than those who lived in older, less adapted buildings.
Hot days have long been known to increase deaths and hospitalizations, but the biological mechanisms underlying these impacts are still poorly understood. “Our bodies are highly adapted to maintaining a stable core body temperature, something our lives depend on,” says Minor.
“Yet every night they do something remarkable without most of us consciously knowing it: they release heat from our core out 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 sleep deprivation, 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 Climate Effects”
The global temperature at the level of the earth’s surface 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 indicates 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.
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 points out that the impact of temperature on sleep should be included in more studies on the possible consequences of global warming. “To make informed future climate policy decisions, we need to take more into account the full spectrum of plausible future climate impacts,” the researcher said.