Humans have evolved to spend roughly a third of our lives sleeping. How we sleep impacts productivity, alertness, mood, hunger, energy, and other basic functions that comprise a huge chunk of the human experience. But we are not sleeping well. The Centers for Disease Control and Prevention reports that 1 in 3 American adults don't get enough sleep, and we're sleeping less than we did about a decade ago. Similar trends are developing across the globe.
Previous studies have pinned some of the blame for our collective sleep problems on technology and noise and light pollution. But Kelton Minor, a doctoral student at the University of Copenhagen's Center for Social Data Science, wondered whether rising nighttime temperatures due to climate change might be contributing to the growing sleep deficit. On Friday, Minor and some colleagues published the largest study ever conducted on the relationship between ambient temperature and sleep. Their findings, published in the science journal One Earth, don't bode well for humans' sleep outlook in a climate-changed world.
Studies have shown that people recall sleeping poorly during hot periods, but, until now, researchers haven't been able to pinpoint what, exactly, is happening to people's sleep patterns during heatwaves. Are they waking up earlier, going to bed later, more restless throughout the night? Fitness wristbands and other wearables, like Apple Watches, clued Minor and his fellow researchers into how, exactly, temperatures affect sleep and what our sleep might look like as climate change accelerates.
By working with an enormous data set — 10 billion sleep observations pulled from 7 million sleep records from 47,000 individuals across 68 countries — and comparing that data to meteorological and climate data, the researchers found that warming temperatures have already eroded 45 hours of sleep per person per year by influencing people to fall asleep later and wake up earlier. That's roughly 10 or 11 additional nights of poor sleep annually. The effects of climate change on sleep start at surprisingly low temperatures, at around 50 degrees Fahrenheit, and grow more severe as temperatures rise. By the end of the century, even if we stabilize greenhouse gas emissions, we'll lose 50 hours of sleep per year, or 13 days of short sleep.
"What we found is that, already, early in the 21st century, right around now, we already estimate that suboptimal ambient temperatures erode quite a lot of sleep," Minor told Grist.
Robbie Parks, a researcher of environmental health sciences at Columbia University who was not involved in this study, called it a "landmark" addition to the field of climate change and sleep. "Using rigorous methodology and an unprecedented dataset of human wearables around the globe, Minor and colleagues have demonstrated how the loss of sleep due to rising temperatures could worsen everywhere under future climate projections," Parks said. "Lack of sleep can have far-reaching consequences for human health, worsening physical and mental health, potentially resulting in injuries such as transport accidents, and also potentially increasing aggression and deaths of despair," he added.
As with most climate change-related impacts, the effects of rising temperatures on sleep will not be felt by everyone equally. Minor's study shows that people in low-income countries, the elderly, and women are experiencing bigger sleep impacts from climate change. The researchers aren't sure why, exactly, that is, though they have some theories. Reduced access to cooling technology like air conditioning could be a factor in why people in lower-income countries are three times more impacted by higher nighttime temperatures than people in higher-income nations.
Minor's research shows that, for the elderly, sleep quality is twice as impacted per degree of warming. That could be because thermal sensitivity increases with age. For cisgender women, core body temperature decreases earlier in the night than it does for cis men, which means they go to sleep earlier on average and may be exposed to higher temperatures as they're preparing for bed.
Howard Frumkin, former Dean of the UW School of Public Health and senior vice president of the Trust for Public Land, who was not involved in the study, said the research is an advancement over previous studies. He wasn't surprised that climate change affects the way people sleep unevenly. "While the paper didn't provide data, it's likely that poor people here in the U.S. are disproportionality affected too," Frumkin told Grist. "In fact, the study was likely skewed toward relatively wealthy people (that's who wears fancy wristbands) so it may underestimate the impact of heat on sleep."
Minor was surprised to discover that people in warmer climates were more likely to experience sleep loss as temperatures rise than people in colder climates. He had assumed that people living in warmer places are already acclimated to hot temperatures and wouldn't have trouble adjusting to a new normal. That wasn't the case. "This really suggests that there's limited evidence of adaptation," he said. In other words, it looks like we're not going to get used to sleeping in hotter temperatures any time soon.
Limiting climate change as much as possible will also limit the number of nights of short sleep we'll experience by the end of the century. But some of that warming is already baked in. That doesn't mean that climate change's worsening impacts on sleep are inevitable, though. The fact that individuals' sleep in richer countries is less impacted by climate change than the sleep of those in lower and middle-income countries indicates that there is a way to limit the impact of rising temperatures on sleep. More research needs to be done, Minor said, but it's a good bet that expanding access to cooling technologies like air conditioning, planting more trees in urban areas to prevent streets and houses from absorbing so much heat, and lifting people out of poverty could all contribute to better sleep.
"It provides a path forward," Minor said, "as long as cooling is provided equitably."