The climate crisis is making nights hotter. That’s hurting our sleep

You know the feeling…tossing and turning because it’s just too hot to sleep.

“We need to actually cool our bodies down to sleep,” said David Eisenman, professor of Medicine and Public Health at UCLA who studies the health impacts of extreme heat. "So if we can't cool our bodies down, our sleep becomes disturbed. That then has effects on our heart, on our immune system, on our psychology. Our inability to sleep is bad for our bodies and our minds.”

Eisenman said emerging research is showing hotter nights may even increase the risk of stroke, particularly in older adults.

“The idea here is that we think that persons are getting dehydrated at night because it's too warm in their homes. This increases the thickness of their blood and this leads to greater risk of blood clot formation and then strokes occur,” he said.

Eisenman added that research is also showing that the overnight temperature associated with the biggest risks is around 80 degrees, but the threshold will be different for different people, and can depend on other factors such as levels of humidity.

Some feel the nighttime heat more than others

Wealthier households can often protect themselves from this worsening trend because they live in better-insulated housing and can afford to run air conditioning for longer periods of time.

But lower-income communities largely home to people of color are disproportionately affected by both the urban heat island effect and climate change that's contributing to hotter nighttime temperatures. Historically redlined communities with fewer trees and more concrete and pollution are particularly at risk.

The risk of health impacts caused by disturbed sleep is particularly high for people without air conditioning ( some 20% of homes in Los Angeles have no AC, for example).

Even for those who do have air conditioning, many with lower or fixed incomes can’t afford to run it long or at all . And as more people get pushed out of safe housing — largely due to unaffordable rent and housing costs — and onto the streets or into mobile homes that can’t sufficiently keep heat out, the health affects will only grow.

Eisenman said better public health messaging around the specific affects of nighttime heat is needed.

“We've seen a lot of good messaging around high temperatures and it always sort of assumes daytime temperatures….here's where you can go during the day if it's really hot,” Eisenman said. “We aren't dealing with the nighttime temperatures in any of our risk messaging that comes out of governments or public health departments, so this is really an important gap in our approach. We need to think about how can we provide resources for nighttimes.”

Longer-term solutions include increasing shade and cool roofs and pavement in the hottest neighborhoods, as well as access to energy-efficient cooling technology such as heat pumps . Cutting global carbon pollution is key to preventing the issue from getting worse.

What hotter nights say about climate change

The science is clear that greenhouse gasses pumped into the atmosphere by human society are driving hotter and longer heat waves around the world, including here in the Southland.

And nights are warming nearly twice as fast as days .

That trend is hitting home in Southern California, not just because it’s summer and hotter temperatures are exacerbating the urban heat island effect, said Alex Tardy, a meteorologist with the National Weather Service (NWS) San Diego office.

It’s largely human-caused climate change, he said.

“The heat waves are getting worse. The heat waves are containing a little bit more moisture. So that means your nighttime temperatures are also warmer,” Tardy said.

Warmer air holds more moisture, Tardy explained. As the climate crisis makes heat waves hotter and longer, additional moisture is sucked into the atmosphere. (That also dries out vegetation faster, raising the risk for more severe wildfire).

To make matters worse, more moisture in the atmosphere can also mean more humid weather — that’s what we’re seeing in the current heat wave, Tardy said. While that humidity can keep daytime temperatures lower, it makes nighttime temperatures higher, he added.

“It's kind of a lose-lose scenario because your daytime is not quite as hot, but your nighttime is equally or even slightly warmer,” Tardy said. “So we're going to be spending a lot of our time in the 80s and 90s in a lot of these inland locations, even more than what we just recently did [in the late June and early July heat waves].”

More humidity makes it even harder for our bodies to cool down, since sweating isn’t as effective, further exacerbating the risk of heat illness.

How far from normal are we?

Tardy said here in the Southland, nighttime temperatures in the mountains and deserts have been 4 to 8 degrees above normal for this time of year — for example, Big Bear has seen nighttime temperatures 6 degrees above normal for July. Palm Springs has been 8 degrees above normal, with nighttime lows averaging 87 degrees, instead of the long-term average for this time of year of 79 degrees.

The coast is starting to see above-normal nighttime temperatures now too, as the effects of May gray and June gloom have burned off.

“The trend is that you're having these anomalies repeat,” Tardy said. “Now you're not always first place, you're not always second place, but it's just like looking at sports — if a team wins the division six out of 10 years, there's a definite trend.”

Six out of the last 10 years have ranked among the top 10 highest for average night temperatures in Palm Springs, for example, Tardy said.

“Whenever you have anomalies, you don't want them to be showing up year after year or month after month,” he said. “Part of how we look at climate change is what's driving the overnight temperatures. It's not just urban heat island, it's also a planet or atmosphere that potentially has more moisture. And the reason why it has more moisture is because it's overall the air mass is warmer. So warmer air will hold more moisture.”