If you’ve ever laid awake at night wondering about how bad wildfires, droughts, storms, and floods could get in the future, then climate change has already been robbing you of sleep.
Even for those without eco-anxiety though, warming global temperatures might me making it harder to catch zzz’s, according to a new study published in the journal One Earth. And, like basically all climate change impacts, sleep loss is felt unevenly worldwide, with those in the poorest countries likely to be most negatively effected.
The researchers analysed data from sleep-tracking wristbands, like Apple watches, between 2015 and 2017, in conjunction with weather and climate data. They found that people fall asleep later and wake up earlier when nighttime temperatures are hotter. On nights warmer than 30 degree Celsius, sleep declines by about 14 minutes, on average. And temperatures at or above 25 C boost the chances of a person getting less than 7 hours of sleep by 3.5%, compared with a baseline of 10 C.
Using a combination of climate models and observed data, the scientists extrapolated those sleep reductions to estimate that, as of 2010, people were losing an average of 44 hours of sleep and experiencing 11 nights of sleep deprivation annually because of climate-change related, hot temperatures.
“Our results imply that suboptimal ambient temperatures likely already erode human sleep considerably early in the 21st century,” wrote the study authors in the new publication. By the end of this century, they projected those numbers could go up to an average of 58 lost hours and more than 14 days of short sleep per year, under the worst warming scenarios.
Previous studies have suggested a correlation between rising global temperatures and increasing sleep loss, including research from the same lead scientist. However, these past studies have mostly depended on self-reported data, which can be fraught as people aren’t the most reliable at accurately describing their own experiences.
In this new research, the scientists instead went to the actual, tracked data that health-monitoring wristbands can offer. They included data from 47,628 people in 68 different countries.
Within that sizable sample, the scientists further found that some people were more vulnerable to hotter temperatures than others. Women were slightly, but significantly, more likely to experience higher levels of sleep loss than men. People over the age of 65 experienced twice the amount of sleep loss, per degree Celsius of temperature increase. Those in warmer regions also lost more sleep per degree of temperature rise. And people living in poorer countries incurred the largest effect of hot nights. Residents in lower-middle-income countries lost almost three times as much sleep compared with those in high-income countries.
“Taken together, our results demonstrate that temperature-driven sleep loss likely has and may continue to exacerbate global environmental inequalities,” write the study authors.
Some previous research has suggested that people can adapt and get used to hot temperatures. However, in the new study, the scientists found no evidence of people’s sleep loss patterns changing over the course of a season: hot temperatures in early summer were just as detrimental to sleep as in late summer, and people in warm climates weren’t less prone to temperature-related sleep loss (in fact, the opposite was true). Based on the new study, people don’t seem to be able to adapt to heat, past a certain point.
Without enough sleep, people suffer all sorts of mental and physical consequences: from depression to heart disease. So, climate change’s effects on sleep are really climate change’s effects on human health. Minutes of lost sleep might not seem like much, but cumulatively, over time it could have a negative impact.
Because the study relied on climate models, there’s a degree of uncertainty, and the scientist’s future predictions are likely to contain error. But, most ominously, the study authors warn that their findings could be a big underestimation of the problem.
Health-tracking wristband data, although probably more reliable than self-reporting, comes with its own limitations. Most of the people with these sorts of tech gadgets are from high-income countries, and the scientists reported that their dataset also skewed middle aged and male (when, again, women, older people, and those from poor countries seem to be most heavily impacted).
Even within lower-income countries, those with high tech wearables are likely to be wealthier than others. Wealthier people are more likely to have access to things like air conditioning, fans, and other adaptive measures. Plus, the hottest, tropical regions of the world were poorly represented in the dataset.
On top of sussing out those biases in the data, the authors suggest that future research should focus on equitable policies and design — or, new ways to help people adapt to the heat. Climate change is here, the temperature is going up, and we’ll have to figure out how to rise to the occasion.