Living in South Australia, the hottest and driest of Australia’s six states, and feeling the brunt of the region’s increasingly intense seasonal heatwaves, local researchers wanted to know more about the impact of our warming world on human sleeping patterns.
In late January, the South Australian city of Adelaide endured its hottest night on record, with thermometers still measuring temperatures of 34.1° Celsius (about 93° Fahrenheit) just before 7 am. Such sweltering nighttime conditions have long been linked to a greater risk of hospitalization for everything from cardiovascular to mental health conditions. “We started wondering whether some of this could be explained by loss of sleep, or poor quality sleep,” says Bastien Lechat, a scientist leading a research program on sleep health at Flinders University.
The group’s work has already shown that around the world, higher nighttime temperatures impact both the number of hours of sleep we get and the regularity of our sleeping patterns. But most concerningly of all, they’ve found that it can trigger sleep apnea, a chronic condition in which people stop breathing dozens or even hundreds of times during the night, which itself has been linked to high blood pressure, diabetes, heart disease, dementia, and a greater risk of road accidents.
Sleep apnea is already estimated to impact around a billion people around the world, many of whom are unaware they have the condition. But if global warming progresses as expected, research carried out by Lechat and others has shown that this number may not only increase, but the severity of the sleep apnea events could worsen.
One particularly eye-opening study, carried out by giving under-mattress sleep sensors to 67,558 people across 17 European countries and recording their sleep data over the course of five summers, found that the prevalence of sleep apnea events increased by 13 percent at the peak of a heatwave. It also revealed that for every 1°C increase in the nighttime temperature, the rates of sleep apnea events rose by 1.1 percent, with the risk being even greater during especially humid nights.
With the average global temperature projected to increase to somewhere between 2.1 and 3.4° C above pre-industrial levels by the year 2100, the group’s research predicts that there could be a 1.2 to 3-fold increase in the prevalence of sleep apnea events by the end of the century. Lechat commented that the impact will be felt even more harshly in low-income populations without access to air conditioning.
“These results are an important wake-up call,” says Lechat. “What is concerning too is that this increase in sleep burden will disproportionately worsen existing health disparities. For those with lower socioeconomic status, we may have underestimated the true effect. For example, a recent study in the US showed that the effect of heat on sleep duration was 10 to 70 percent greater among Hispanic populations.”
It isn’t only Australian sleep experts who have been paying attention to this threat. Two years ago, Chinese scientists used smartwatch data collected from 51,842 people across 313 cities over the course of three years and found that every 10° C increase in daily temperature resulted in a 8.4 percent increase in sleep apnea events.
There are likely a number of reasons for this correlation. High temperatures impair the body’s ability to cool down at night, meaning that sleep is lighter and more fragmented, and sleep apnea events tend to occur more during light rather than deep sleep. Studies also show that hot weather can influence behavior, making people more likely to drink more alcohol, consume a poorer diet, and be less physically active, changes which are all known risk factors for sleep apnea.