Add climate change to the list of reasons we can't sleep at night. And it's not just because we're worrying about the future of the planet.

You don't sleep as well when night temperatures are warmer than normal, researchers have found. And if climate change isn't addressed soon, in the next few decades, the problem could mean millions of more nights of poor sleep each year.

Nick Obradovich was a doctoral student in political science at the University of California San Diego when a heat wave hit the city in October 2015. He tossed and turned all night and couldn't sleep, getting little help from his air conditioner. Obradovich noticed his fellow students on campus were grumpy and lethargic and wondered if anyone had made the connection between sleep and climate change. He decided to see if there was a link.

Calculating sleep loss

Obradovich, now a fellow at the Harvard Kennedy School and a research scientist at the Massachusetts Institute of Technology's Media Lab, and his team used data from a Centers for Disease Control and Prevention survey of 765,000 Americans. Among the many questions they were asked: “During the past 30 days, for about how many days have you felt you did not get enough rest or sleep?”

The authors cross-referenced those responses with weather station temperature information to see if the respondents had been exposed to unusually warm temperatures when they were trying to sleep.

Using this information, they calculated that every nighttime increase of 1 degree Celsius equaled three nights of poor sleep per 100 people each month. The authors wrote in their Scientific Advances study, this, "if extrapolated across the current population of the United States, would produce nearly 9 million additional nights of insufficient sleep per month or approximately 110 million extra nights of insufficient sleep annually."

The negative effects are more severe in summer and are felt more acutely in people over 65 and for those who make less than $50,000 each year. Older people are more susceptible to heat-related problems, and those in lower-income groups may not be able to afford air conditioning.

"Sleep has been well-established by other researchers as a critical component of human health. Too little sleep can make a person more susceptible to disease and chronic illness, and it can harm psychological well-being and cognitive functioning," Obradovich said in a statement. "What our study shows is not only that ambient temperature can play a role in disrupting sleep but also that climate change might make the situation worse by driving up rates of sleep loss."

Looking to the future

climate change sleep loss map predictions Predicted effects of climate change–induced nighttime warming on human sleep. (Photo: Obradovich et al/Scientific Advances)

And what will happen if temperatures continue to rise?

The researchers used climate projections by NASA Earth Exchange for 2050 and 2099 and found that the climate change/sleep link will only get worse. They said warming temperatures could cause an extra six nights of problematic sleep for every 100 people by 2050 and about 14 extra nights of insufficient sleep by 2099.

"The U.S. is relatively temperate and, in global terms, quite prosperous," Obradovich said. "We don't have sleep data from around the world, but assuming the pattern is similar, one can imagine that in places that are warmer or poorer or both, what we'd find could be even worse."

Mary Jo DiLonardo writes about everything from health to parenting — and anything that helps explain why her dog does what he does.