Researchers used wristbands with internal accelerometers to measure sleep duration and sleep timing in over 47,000 adults across 68 countries for an average of six months for a study published in the journal One Earth.
Adults should get seven to nine hours of sleep, according to the National Sleep Foundation. The likelihood of getting less than seven hours of sleep increased by 3.5% if minimum outside nighttime temperatures exceeded 77 degrees Fahrenheit (25 degrees Celsius) compared with the baseline temperature of 41 to 50 degrees Fahrenheit (5 to 10 degrees Celsius), the study found.
“The 3.5% sleep loss may initially look like a small number, but it adds up,” said Alex Agostini, lecturer in the department of justice and society at the University of South Australia in Adelaide. She was not involved in the study.
When adults do not receive the recommended amount of sleep, they may have issues with concentration, Agostini said. Long-term effects can include an increased risk of some health concerns like cardiovascular and gastrointestinal diseases, she said.
“Many of us have pulled all-nighters at some stage of our lives — imagine doing this eight times. How would you feel?” Agostini said.
A single night over 86 degrees Fahrenheit (30 degrees Celsius) reduces sleep time by about a quarter hour per person, said lead study author Kelton Minor, doctoral candidate at the Copenhagen Center for Social Data Science at the University of Copenhagen.
However, the elderly lost twice the amount of sleep per degree of warming compared with young or middle-aged adults. What’s more, sleep loss was three times larger for seniors in lower-income areas compared with higher-income areas, he said. Women were also about 25% more affected by the rising temperatures than men, Minor added.
Low chance of adapting to the heat
Researchers also found evidence that people living in warmer climates lose more sleep per degree compared with those in colder climates, and that people are better at adapting in colder climates than in hotter ones. This greater sleep loss in warmer places suggested that people cannot easily adapt to warmer temperatures, Minor said.
As temperatures continue to rise due to global warming, Minor projected that sleep loss will increase at a faster rate in regions that already face hot climates compared with those that don’t.
People didn’t seem to adjust to the heat, either: The amount of sleep people received the first month of summer when people were less familiar with the heat and the last month of summer when people were more familiar with it showed they lost almost the same amount of sleep, according to Minor.
This similarity in sleep loss indicated that people cannot adapt to higher temperatures over time, he said. Additionally, results showed that people did not appear to recover the sleep they lost during a warm night in the two weeks after a temperature spike, Minor said.
The cost of warmer temperatures
Humans spend about a third of their lives asleep, yet a growing number of people do not sleep enough, Minor said. One-third of adults in the United States report that they typically get less sleep than the recommended seven to nine hours, according to the US Centers for Disease Control and Prevention.
“Many of us already do not get enough sleep, and the contribution of sleep issues relevant to global warming could have real consequences for our health and wellbeing,” Agostini said via email.
When humans begin to go to sleep, their core body temperature drops, she said. When the surrounding temperature is warmer, it makes it harder to cool down, which can impact the ability to fall asleep, she explained.
Air conditioning may allow people to adapt to the warmer temperatures, but it’s not a reliable, long-term solution, Agostini said.
People living in lower-income countries have less access to air conditioning, which could further the equality divide, Minor said.
In addition, air conditioners release greenhouse gas emissions, which naturally increase global warming, Agostini said.
“The bigger and better solution to the problem is the use of environmentally friendly building planning and implementing other changes to improve the issue of global warming,” she said.