Rising Temperatures Are Taking a Toll on Sleep Health

Living in South Australia, the hottest and driest of Australia’s six states, and feeling the brunt of the region’s increasingly intense seasonal heat waves, 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 degrees Celsius (about 93 degrees 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 degree Celsius 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 degrees Celsius above preindustrial levels by the year 2100, the group’s research predicts that there could be a 1.2- to threefold 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 degrees Celsius 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.

There’s also the physiological effect of heat itself on our breathing. “Heat may also destabilize breathing control, increase fluid retention, and promote dehydration, all of which can make the upper airway more collapsible and increase the likelihood of sleep apnea,” says Lucia Pinilla, another researcher at Flinders University investigating the subject.

At the same time, sleep apnea is already expected to become a growing problem for those living in cities, due to the chronic impact of air pollution on nighttime breathing, something which is only predicted to get worse. Last year, Hong Kong researchers led a study where they found that both short and long-term exposure to PM2.5 particles—tiny airborne particles, less than 2.5 micrometers wide, which are generated by sources such as vehicle exhausts, factory emissions, and wildfires and can penetrate deep into the lungs and bloodstream—makes sleep apnea more likely.

Others have shown that the same is true for nitrogen dioxide, a reddish brown gas released into the air from exhausts, power plants, and other industrial facilities, while exposure to pollution also worsens symptoms for people with existing sleep apnea.

“Obstructive sleep apnea occurs when the upper airway becomes unstable and collapses during sleep,” says Martino Pengo, associate professor from the University of Milano-Bicocca who studies the subject. “Air pollutants can increase this instability by irritating the tissues of the throat, making the airway narrower and more prone to collapse when muscle tone naturally falls at night. Nitrogen dioxide is a strong airway irritant and may promote local inflammation that can fragment sleep and destabilize breathing.”

While concerning, such research also points to possible ways of mitigating risk. Martha Billings, professor of medicine in the University of Washington’s division of pulmonary, critical care, and sleep medicine advises using indoor air filtering devices if online air quality databases show that your city ranks particularly poorly. “I would recommend it especially if the air quality index is greater than 200 as can happen with forest fires or other stagnant air,” she says.

Likewise with rising temperatures, losing weight could be a way of mitigating your sleep apnea risk. Research has highlighted how those who are overweight or obese are more at risk of experiencing more severe episodes of sleep apnea in hot weather, with their breathing stopping and starting more often during the night. Pinilla says this relates to how accumulating body fat alters the body’s internal temperature regulation. “People with higher fat mass tend to retain more heat, and dissipate it less efficiently making it harder to maintain a comfortable core temperature at night,” she says. “On hot nights, this can lead to lighter and more fragmented sleep.”

Men, who are more vulnerable to sleep apnea anyway as they are more prone to accumulating fat in the neck and have a longer and more collapsible upper airway, are particularly at risk, as are those with preexisting mental health or sleep disorders.

The Flinders team is hoping to ultimately be able to develop personalized advice and heat-warning systems for those who are at greater risk of experiencing sleep apnea events during heatwaves, as well as simple solutions which anyone can turn to. They are hoping to gain funding to be able to run experiments where people receive cooling mattress toppers or follow specific behavioral advice, such as ensuring they are well hydrated when going to bed, to see if this can actively prevent apnea events in warmer weather.

“Ultimately, our goal is to develop practical, evidence-based recommendations that can be applied during heat waves, particularly for vulnerable groups and people at higher risk,” says Pinilla.