In July 2016, Boston experienced a devastating heat wave, with daytime temperatures averaging 92 degrees for five consecutive days. Some local university students who stayed in the city over the summer were lucky and lived in dorms with central air-conditioning. Other students were not so fortunate — they were stuck in older dorms with no AC
José 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, affects the cognitive performance of young adults. He asked 44 students to take math and self-control tests five days before the temperature rose, every day during the heat wave, and two days after.
“Many of us think we're immune to heat,” said Dr. Cedeno, an assistant professor of environmental and occupational health and justice at Rutgers University. “So I wanted to test whether that's actually true.”
It turns out that even young, healthy college students are affected by high temperatures. During the hottest days, students living in dorms without air-conditioning, where nighttime temperatures averaged 79 degrees, performed significantly worse on tests held each morning than students with AC, whose rooms remained a pleasant 71 degrees.
The Northeast, South and Midwest are once again experiencing a heat wave. High temperatures can have dangerous effects on our bodies, increasing the risk of heart attack, heat stroke and death, especially among the elderly and people with chronic illnesses. But heat also takes a toll on our brains, impairing cognition and making us irritable, impulsive and aggressive.
How heat makes us dumb
Several studies conducted in the laboratory have produced similar results to Dr. Cedeno's research, with scores on cognitive tests dropping when scientists raised the room temperature. One investigation found that an increase of just four degrees – which participants reported still feeling comfortable – led to an average 10 percent drop in performance on tests of memory, reaction time and executive functioning.
This could have real consequences. R. Jisung Park, an environmental and labor economist at the University of Pennsylvania, looked at high school standardized test scores and found that they fell 0.2 percent for every degree above 72 Fahrenheit. That might not seem like much, but for students taking a test in a non-air-conditioned room during a 90-degree heat wave, it could be a lot.
In another study, Dr. Park found that the more hot days there were during the school year, the worse students performed on standardized tests — especially when the thermometer climbed above 80 degrees. She believes this may be because increased exposure to heat was affecting students' learning throughout the year.
Dr Park said the effect was “more pronounced for lower-income and racial minority students”, possibly because they had less access to air-conditioning both at school and at home.
Why does heat make us aggressive?
The researchers first discovered the link between heat and aggression by looking at crime data, finding that hotter days tend to have more homicides, assaults, and domestic violence incidents. The relationship also applies to nonviolent acts: When temperatures rise, people are more likely to post hate speech online and honk their horns in traffic.
Lab studies confirm this. In a 2019 experiment, people behaved more maliciously toward others when playing specially designed video games in a warm room than in a cool room.
So-called reactive aggression is particularly sensitive to heat, possibly because people perceive the actions of others as more hostile on hot days, and are therefore more inclined 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 heat's effects on cognition, specifically a decline in self-control. “Your tendency to act without thinking, or not being able to stop yourself from acting in a certain way, these things are also affected by heat,” she said.
What is happening in the brain?
Researchers don't know exactly why heat affects our cognition and emotions, but there are some theories about it.
One is that brain resources are being diverted to keeping you cool, leaving less energy for everything else. “If you're allocating all the blood and all the glucose to the parts of your brain that are focused on thermoregulation, it seems like it's very likely that you won't have enough energy left for these kinds of higher cognitive functions,” Dr. Meidenbauer said.
You may also become distracted and irritable because of the heat and misery. It turns out that this is actually one of the brain's response responses. If you can't calm down, your brain “will make you feel even more uncomfortable, so that it will be much more difficult for you to find the things you need to survive,” explained Sean Morrison, a professor of neurological surgery at Oregon Health & Science University.
The effect of heat on sleep may also play a role. In the Boston study, the hotter it was, the more disrupted students' sleep was — and the worse they performed on tests.
The best way to minimize these effects is to cool yourself down as quickly as possible. If you don't have access to air-conditioning, fans can help, and make sure you stay hydrated. It may seem obvious, but the most important thing for your brain, mood, and cognition is how hot your body is, not the temperature outside.