In July 2016, a heat wave hit Boston, with average daytime temperatures reaching 92 degrees for five consecutive days. A few local university students staying in town for the summer were lucky and stayed in dormitories with central air conditioning. Other students, not so much – they were stuck in older dorms with no AC
Jose Guillermo Cedeño Laurent, a Harvard researcher at the time, decided to take advantage of this natural experiment to see how heat, and especially heat at night, affected the cognitive performance of young adults. He had 44 students take math and self-monitoring tests five days before the heat, every day during the heat wave, and two days after.
“Many of us think we’re 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 if that was really true.”
It turns out that even young, healthy college students are affected by high temperatures. On the hottest days, students in the non-air-conditioned dorms, where nighttime temperatures averaged 79 degrees, performed significantly worse on the tests they took each morning than students with AC, whose rooms stayed at 71 degrees.
A heat wave is once again covering the Northeast, South and Midwest. High temperatures can have an alarming effect on our bodies, increasing the risk of heart attacks, heatstroke and death, especially among the elderly and those with chronic conditions. But heat also affects our brains, reducing cognitive function and making us irritable, impulsive and aggressive.
How heat damages our cognition
Numerous studies in laboratory settings have produced similar results to Dr. Cedeño’s research, with scores on cognitive tests decreasing as scientists raised the temperature in the room. One study found that just a four-point increase—which participants described as still feeling comfortable—resulted in an average 10 percent drop in performance on tests of memory, reaction time, and executive function.
This can have real consequences. R. Jisung Park, an environmental and labor economist at the University of Pennsylvania, looked at standardized high school test scores and found that they dropped 0.2 percent for every degree above 72 Fahrenheit. That might not sound like much, but it can add up for students taking exams in a non-air-conditioned room during a 90-degree heat wave.
In another study, Dr. Park found that the warmer-than-average days during the school year, the worse students did on a standardized test — especially when the thermometer climbed above 80 degrees. He believes this may be because the greater heat exposure was affecting students’ learning throughout the year.
The effect was “more pronounced for low-income and racial minority students,” Dr. Park said, possibly because they were less likely to have air conditioning, both at school and at home.
Because heat makes us aggressive
Researchers first discovered the link between heat and aggression by looking at crime data, finding that there are more murders, assaults and incidents of domestic violence on hot days. The connection also applies to non-violent acts: When temperatures rise, people are more likely to engage in hate speech online and honk their horns in traffic.
Laboratory studies confirm this. In a 2019 experiment, people behaved more viciously toward 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, likely because people tend to interpret the actions of others as more hostile on hot days, prompting them to respond in kind.
Kimberly Meidenbauer, an assistant professor of psychology at Washington State University, believes this increase in reactive aggression may be related to the effect of heat on cognitive function, particularly a decline in self-control. “Your tendency to act without thinking or not being able to stop yourself from behaving in a certain way, these things also seem to be affected by heat,” he said.
What happens in the brain
Researchers don’t know why heat affects our cognition and emotions, but there are a few theories.
One is that brain resources are diverted to keeping you cool, leaving less energy for anything else. “If you distribute all the blood and all the glucose to parts of your brain that focus on thermoregulation, it seems very plausible that you simply wouldn’t have enough left for some of these higher cognitive functions. ,” said Dr. Meidenbauer.
You could also become distracted and irritable because of how hot and miserable you feel. It turns out that this is actually one of the brain’s coping responses. If you can’t relax, your brain “will make you feel even more uncomfortable, so you consume everything you need to survive,” explained Shaun Morrison, a professor of neurological surgery at Oregon Health & Science University.
The effect of heat on sleep could also play a role. In the Boston study, the hotter it got, the more disturbed the students’ sleep—and the worse they performed on tests.
The best way to counteract 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 may sound obvious, but what matters most to your brain, mood, and cognition is how warm your body is, not the outside temperature.
