Nights in Wisconsin cities have become hotter. The cost is beyond just loss of sleep.

No one ever thought of Wisconsin as being in the tropics, but a new report says our cities are experiencing more uncomfortably hot "tropical nights," raising the risk of a cascade of consequences from a loss of sleep.

Climate Central, a nonprofit organization that reports on climate science, found that countries around the world are seeing a growing number of nights during which the temperature does not fall below 68 degrees Fahrenheit. That's above what medical experts say is the optimal range of temperatures for getting a good night's sleep.

The stakes are high — prolonged sleeplessness can cause brain fog, worsen anxiety and depression, and even increase risk of heart attack and stroke. Experts say this is one more example of the effects of human-caused climate change, and that vulnerable populations will feel them disproportionately.

"Those of us who have the means, we can cool our nighttime environment," said Dr. Lisa Cottrell, a clinical health psychologist in Milwaukee who specializes in sleep issues. Cottrell is a member of Healthy Climate Wisconsin, a group of health professionals advocating for climate action. "Those of us who don't have the means to do that, there isn't anything they can do."

The Climate Central analysis compared real-life nighttime temperatures over the last decade to a model estimating the nighttime temperatures that would have occurred absent the effects of climate change. The difference between those two numbers is what they deemed the number of additional hot nights due to climate change.

In Wisconsin:

• Appleton experienced an average of 16 more uncomfortably hot nights per year in the last decade.
• Madison, Kenosha and La Crosse experienced an average of 14 more hot nights.
• Green Bay experienced an average of 12 more hot nights.
• Milwaukee, Eau Claire and Racine experienced an average of 11 more hot nights.

Extreme heat interrupts the body's sleep cues

Intuitively, it's easy to picture why a hot night could become a sleepless one. It's hard to get comfortable enough to doze off. The U.S. Centers for Disease Control and Prevention recommends a "comfortably cool" room temperature of about 65 to 68 degrees Fahrenheit to produce good sleep.

Our bodies actually rely on the temperature lowering to tell our brains that it's time to go to sleep, said Dr. Anne Getzin, a family medicine physician in Milwaukee and member of Healthy Climate Wisconsin. If the night is too warm, Getzin said, the brain won't receive the signal that makes way for good-quality sleep. In addition, if the body has to continue working overnight to cool itself down, it won't have time for rest and recovery.

That's in part why hot nights can pose more of a concern than hot days, which tend to make headlines with soaring temps and sunlight beating down, said Climate Central meteorologist Lauren Casey. If the day is hot and people can't get relief from that at night, it can cause a number of physical and mental health problems.

Even in the short term, not getting good quality sleep puts a considerable amount of physical stress on the body, Getzin said. Besides increasing the risk of heart attack and stroke, it can weaken the immune system, making it easier to get sick.

That's on top of things like heat stroke, heat exhaustion and injury to the kidneys from dehydration that happen during heatwaves.

Mentally, "it's the basic stuff" that a lack of sleep impairs, Cottrell said — short-term memory, problem-solving ability, emotional regulation and other mental functions. People's ability to handle daily stressors declines, she said, and those who have anxiety and depression are likely to see their symptoms worsen.

Hot, sleepless nights aren't experienced equally

Historically cooler states like Wisconsin are already uniquely vulnerable to the effects of extreme heat, Getzin said, because our bodies aren't accustomed to higher temperatures and many of our buildings don't have air conditioning.

She pointed to a paper from the National Bureau of Economic Research that suggests that without adaptation measures to account for a hotter world, the Midwest and Great Plains are projected to see the largest impacts to mortality due to their current lack of preparedness for extreme heat.

Like many cities, Milwaukee experiences a phenomenon known as the urban heat island effect, where an abundance of manmade surfaces like parking lots, sidewalks and streets absorb and hold onto heat. That can produce temperatures several degrees hotter than the temperature forecast in certain parts of the city during heat waves, another Climate Central analysis found last year. Punishing heat also tends to affect communities of color more due to historic redlining policies that pushed those residents into more industrial areas with less tree canopy cover.

Without addressing the root causes of climate change by limiting the burning of fossil fuels and switching to cleaner forms of energy, Cottrell said, "it is the most vulnerable people who are going to suffer" from hot, sleepless nights.

Madeline Heim is a Report for America corps reporter who writes about environmental issues in the Mississippi River watershed and across Wisconsin. Contact her at 920-996-7266 or mheim@gannett.com.