Increasing ambient temperatures negatively impact human sleep around the globe, according to a study.
The research, published recently in the journal One Earth, suggests that by the year 2099, suboptimal temperatures may erode 50 to 58 hours of sleep per person per year.
The researchers also found that the effect of temperature on sleep loss is substantially larger for residents from lower income countries as well as in older adults and females.
“Our results indicate that sleep — an essential restorative process integral for human health and productivity — may be degraded by warmer temperatures,” said study first author Kelton Minor of the University of Copenhagen in Denmark.
“In order to make informed climate policy decisions moving forward, we need to better account for the full spectrum of plausible future climate impacts extending from today’s societal greenhouse gas emissions choices,” Minor said.
It’s long been known that hot days increase deaths and hospitalisations and worsen human performance, yet the biological and behavioral mechanisms underlying these impacts have not been well understood, the researchers said.
“In this study, we provide the first planetary-scale evidence that warmer-than-average temperatures erode human sleep,” Minor said. “We show that this erosion occurs primarily by delaying when people fall asleep and by advancing when they wake up during hot weather,” he explained.
The researchers used anonymised global sleep data collected from accelerometer-based sleep-tracking wristbands.
The data included 7 million nightly sleep records from more than 47,000 adults across 68 countries spanning all continents except for Antarctica.
Measures from the type of wristbands used in this study had previously been shown to align with independent measures of wakefulness and sleep.
The study suggested that on very warm nights — greater than 30 degrees Celsius — sleep declines an average of just over 14 minutes. The likelihood of getting less than seven hours of sleep also increases as temperatures rise.
“Our bodies are highly adapted to maintain a stable core body temperature, something that our lives depend on,” Minor said.
“Yet every night they do something remarkable without most of us consciously knowing — they shed heat from our core into the surrounding environment by dilating our blood vessels and increasing blood flow to our hands and feet,” he added.
The researchers noted that in order for our bodies to transfer heat, the surrounding environment needs to be cooler than we are. Early controlled studies in sleep labs found that both humans and animals sleep worse when the room temperature is too hot or too cold.
However, this research was limited by how people act in the real world: they modify the temperature of their sleeping environment to be more comfortable.
In the latest study, the researchers found that under normal living routines, people appear far better at adapting to colder outside temperatures than hotter conditions.
“Across seasons, demographics, and different climate contexts, warmer outside temperatures consistently erode sleep, with the amount of sleep loss progressively increasing as temperatures become hotter,” Minor said.
One important observation was that people in developing countries seem to be more affected by these changes.
It is possible that the greater prevalence of air conditioning in developed countries could play a role, but the researchers could not definitively identify the reason because they did not have data on air conditioning access among subjects.
The researchers also noted that because they uncovered compelling evidence that the impact of warming temperatures on sleep loss is unequal globally, new research should especially consider more vulnerable populations, particularly those residing in the world’s hottest — and historically poorest — regions.