In July 2016, Boston was hit by a heat wave that saw daytime temperatures average 92 degrees for five consecutive days. Some local college students staying in Boston over the summer were lucky enough to live in dorms with central air conditioning. Others, however, were not. They were stuck in old dorms without air conditioning.
José Guillermo Cedeño Laurent, then a researcher at Harvard University, decided to use this natural experiment to study how heat, especially nighttime heat, affects young people's cognitive performance. He had 44 students take math and self-control tests five days before the temperature rise, every day during the heatwave, and then two days after it.
“A lot of us think we're immune to heat,” says Dr. Cedeño, now an assistant professor of environmental and occupational health and justice at Rutgers University, “so what I wanted to test was whether that's actually true.”
It turns out that even young, healthy college students are affected by high temperatures: On the hottest days, students in unair-conditioned dorms where the average overnight temperature was 79 degrees Fahrenheit performed significantly worse on tests they took each morning than their air-conditioned counterparts, whose rooms were kept at a comfortable 71 degrees.
A heat wave is once again sweeping across the Northeast, South and Midwest. Rising temperatures have dire effects on our bodies, increasing the risk of heart attack, heatstroke and death, especially among older adults and those with chronic illnesses. But heat also has a negative effect on the brain, impairing cognitive performance and causing irritability, impulsivity and aggression.
Why heat makes us stupid
Numerous laboratory studies have produced similar results to Dr. Cedeño's, showing that cognitive test scores drop when scientists increase the temperature inside a room. One study found that a temperature increase of just 4 degrees (which subjects said was still comfortable) led to an average 10 percent drop in performance across tests of memory, reaction time, and executive function.
This can have real consequences: When studying high school standardized test scores, R. Ji-soon Park, an environmental and labor economist at the University of Pennsylvania, found that for every degree above 72 degrees Fahrenheit, scores drop by 0.2%. That may not seem like much, but it makes a big difference for students taking exams in unair-conditioned rooms in 90-degree heat.
In another study, Park found that the more hotter-than-average days there were during the school year, the worse students performed on standardized tests, especially when temperatures exceeded 80 degrees. He believes this may be because increased exposure to heat affects students' learning throughout the year.
The effect was “more pronounced among lower-income and racial minority students,” Dr Park said, possibly because they are less likely to have air conditioning at school or at home.
Why heat makes us aggressive
Researchers were the first to discover a link between heat and aggression by examining crime data, finding more murders, assaults, and domestic violence on hot days. This relationship also holds for non-violent acts: As temperatures rise, people are more likely to make hate speech online or honk their horns in traffic.
Lab studies back this up: In one 2019 experiment, people behaved more meanly toward others while playing a specially designed video game in a hot room than when they played in a cool room.
So-called reactive aggression tends to be particularly sensitive to heat, possibly because on hot days people are more likely to interpret the behavior of others as more hostile and respond in kind.
Kimberly Meydenbauer, 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, specifically reducing self-control. “The tendency to act without thinking and not be able to stop certain behaviors also seems to be affected by the heat,” she said.
What's happening in the brain?
Researchers aren't sure why heat affects our cognition and emotions, but they have some theories.
For one, it diverts brain resources from thermoregulation, leaving less energy for other things: “If you're allocating all your blood and glucose to the parts of your brain involved in thermoregulation, it's very likely that you won't have enough energy for these higher cognitive functions,” says Dr. Meydenbauer.
Heat and discomfort might be making you feel distracted or irritated, but it's actually part of your brain's coping response. If you can't cool down, your brain “will make you feel even more uncomfortable and will focus all its energy on finding what you need to survive,” explains Shawn Morrison, M.D., professor of neurosurgery at Oregon Health & Science University.
The effect of heat on sleep may also be a factor: A Boston study found that as temperatures rose, students' sleep was disrupted and their test scores worsened.
The best way to counteract these effects is to cool your body as quickly as possible. If you can't use air conditioning, a fan can help. And don't forget to stay hydrated. It may sound obvious, but what matters most for your brain, mood, and cognition is how hot your body is, not the temperature outside.
