How heat affects the brain

In July 2016, a heat wave hit Boston, with daytime temperatures averaging 92 degrees for five consecutive days. Some local college students staying in town for the summer got lucky and lived in dorms with central air conditioning. Other students not so much: They were stuck in older dorms without air conditioning

Jose Guillermo Cedeño Laurent, a Harvard researcher at the time, decided to take advantage of this natural experiment to see how heat, and especially night heat, affected the cognitive performance of young adults. He asked 44 students to take math and self-monitoring tests five days before the temperature rose, every day during the heat wave and two days after.

“Many of us think we are immune to heat,” said Dr. Cedeño, now an assistant professor of environmental and occupational health and justice at Rutgers University. “So something I wanted to test was whether it was actually true.”

It turns out that even young, healthy college students are affected by high temperatures. During the hottest days, students in non-air-conditioned dorms, where the nighttime temperature averaged 79 degrees, performed significantly worse on tests they took each morning than air-conditioned students, whose rooms remained at a pleasant temperature of 71 degrees.

A heat wave is once again hitting the Northeast, South and Midwest. High temperatures can have an alarming effect on our bodies, increasing the risk of heart attacks, heat stroke and death, particularly among older adults and people with chronic illnesses. But the heat also puts a strain on our brain, compromising our cognitive abilities and making us irritable, impulsive and aggressive.

Numerous studies in laboratory settings have produced results similar to Dr. Cedeño's research, with cognitive test scores decreasing when scientists increase the temperature in the room. One investigation found that just a four-grade increase – which participants described as still feeling comfortable – led to an average 10% decline in performance on tests of memory, reaction time and executive functioning.

This can have real consequences. R. Jisung Park, an environmental and labor economist at the University of Pennsylvania, examined high school standardized test scores and found that they decreased 0.2 percent for every grade above 72 Fahrenheit. It may not seem like much, but it can be helpful for students taking an exam in a non-air-conditioned room during a 90-degree heat wave.

In another study, Dr. Park found that the hotter-than-average days were during the school year, the worse students performed on a standardized test, especially when the thermometer rose above 80 degrees. He said this could be because increased heat exposure affected student learning throughout the year.

The effect was “more pronounced for low-income and racial minority students,” Dr. Park said, perhaps because they were less likely to have air conditioning, both at school and at home.

Researchers first discovered the link between heat and aggression by examining crime data, finding that more murders, assaults and domestic violence incidents occur on hot days. The link also applies to nonviolent acts: As temperatures rise, people are more likely to engage in hate speech online and honk in traffic.

Laboratory studies confirm this. In a 2019 experiment, people behaved more spitefully towards others while playing a specially designed video game in a warm room than in a cool one.

So-called reactive aggression tends to be particularly sensitive to heat, probably because people tend to interpret the actions of others as more hostile on hot days, prompting them to respond in kind.

Kimberly Meidenbauer, assistant professor of psychology at Washington State University, believes this increase in reactive aggression may be related to heat's effect on cognition, particularly declines in self-control. “Your tendency to act without thinking, or not being able to stop yourself from acting in a certain way, those things also seem to be affected by heat,” she said.

Researchers don't know why heat affects our cognition and emotions, but there are a couple of theories.

One is that the brain's resources are diverted to keeping you cool, leaving less energy for everything else. “If you're allocating all your blood and all your glucose to the parts of your brain focused on thermoregulation, it seems very plausible that you don't have enough left over for some of these higher cognitive functions,” Dr. Meidenbauer said.

You may also be distracted and irritable because of how hot and unhappy you feel. It turns out it's actually one of the brain's coping responses. If you can't calm down, your brain “will make you feel even more uncomfortable, so that finding the thing you need to survive becomes a daunting task,” explained Shaun Morrison, a professor of neurological surgery at Oregon Health and Science University.

The effect of heat on sleep could also play a role. In the Boston study, the hotter it was, the more students' sleep was disturbed and the worse their test scores.

The best way to offset these effects is to cool down as soon as possible. If you don't have access to air conditioning, fans can help and make sure you stay hydrated. It might seem obvious, but what matters most for your brain, mood, and cognitive abilities is how warm your body is, not the temperature outside.

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