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Climate change not only has large-scale effects but can also have a strong influence on essential daily activities such as sleep. A scientific team estimates that by 2099, rising temperatures could reduce between 50 and 58 hours of sleep per person per year.
A study led by the University of Copenhagen and published in the journal One Earth indicates that rising ambient temperatures negatively affect sleep in humans around the world, but the effect is substantially greater for residents of low-income countries, as well as for older adults and women.
The study suggests that on very hot nights, with more than 30 degrees Celsius, sleep decreases by an average of just over 14 minutes. The probability of sleeping less than seven hours also increases as temperatures rise.
"In this study, we provide the first planetary-scale evidence that warmer-than-average temperatures erode human sleep," said first author Kelton Minor of the University of Copenhagen. That effect occurs mainly by delaying the time people fall asleep and advancing the time they wake up in hot weather.
The researchers used anonymous global sleep data collected from accelerometer-based sleep-tracking wristbands. The data included 7 million nightly sleep records from more than 47,000 adults from 68 countries.
During the night, our body gives off heat to the surrounding environment, dilating our blood vessels and increasing blood flow to the hands and feet, and for this, the surrounding environment must be cooler than we are.
The study suggests that on very hot nights (above 30 degrees Celsius, or 86 degrees Fahrenheit), sleep decreases by an average of just over 14 minutes. The probability of sleeping less than seven hours also increases as temperatures rise.
"Our bodies are highly adapted to maintaining a stable core body temperature, something our life depends on," Minor explains. "Yet every night they do something extraordinary without most of us consciously knowing it: they expel heat from our core to the surrounding environment by dilating blood vessels and increasing blood flow to the hands and feet." He adds that for our body to transfer heat, the surrounding environment must be cooler than we are.
The researchers found that, under normal living conditions, people seem to adapt much better to cooler outside temperatures than to warmer ones. "In all seasons of the year, across all demographic groups, and indifferent climatic contexts, warmer outdoor temperatures systematically erode sleep, and sleep loss increases progressively as temperatures rise," Minor said.
An important observation was that people in developing countries seem to be more affected by these changes. The higher prevalence of air conditioning in developed countries may have something to do with it, but the researchers couldn't definitively identify the reason because they didn't have data on access to air conditioning among subjects.
The researchers also note that since they found convincing evidence that the impact of warming temperatures on sleep loss is uneven around the world, further research should take particular account of the most vulnerable populations, especially those residing in the world's warmest and historically poorest regions. (Climate change sleep)
In the future, the team hopes to collaborate with global climate scientists, sleep researchers, and technology providers to extend the reach of global sleep and behaviour analyses to other populations and settings.
In addition, they are interested in studying the impact of increased outside temperatures on sleep outcomes for incarcerated populations located in hot climates, who may have especially limited access to air conditioning.
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