If you're feeling a bit brain-fogged these days, you might not be wrong to blame it on the heat.
Several summers back, researchers in Boston studied young adults living in college dorm rooms during a heat wave. Some had central AC and slept at a cool 71 degrees Fahrenheit. Others slept in rooms without air-conditioning, where the temperature hovered around 80 degrees.
Each morning for nearly two weeks, the students took a few tests, administered on their cellphones. The people who slept in the hotter dorm rooms performed measurably worse on the tests.
The tests included a math test requiring simple addition and subtraction and a second test, the Stroop test, that jumbles colors and words. "So, if I show the word 'red' in the color blue, participants have to respond 'blue,'" says study author Jose Guillermo Cedeño Laurent, an assistant professor at the Rutgers School of Public Health.
It's easy to get tripped up if your attention or reaction time is slowed, he says, and that's exactly what heat appears to be doing. "The magnitude of the effect was really striking," Cedeño Laurent says. "We saw reductions in the order of 10% in their response times and also their accuracy."
Part of this effect may be explained by interrupted sleep. It can be hard to get a good night's rest if you're not accustomed to the heat, and a lack of sleep could certainly impair reaction time and focus. But there's a body of evidence suggesting it may be something about the heat itself that interferes with cognition.
A similar study published in 2021 also documented a dip in cognitive performance at air temps of 79 degrees. Researchers found that as the temperature rose, activity in the parasympathetic nervous system, the anti-stress system that can help us stay calm and relaxed, was lowered. Plus oxygen saturation levels in the blood were lower at the elevated temperatures as well, which the researchers said could be expected to result in reduced cognitive performance.
Other studies have found an effect from heat on office workers and on standardized test score performance, says Caleb Dresser, an emergency medicine physician who also serves as the director of health care solutions at the Harvard Chan Center for Climate, Health and Global Environment.
One of these studies showed that productivity in the workplace is highest when the air temperature is about 72 degrees, and productivity starts to drop off in the mid-70s. Another shows that for high school students, taking a standardized test on a hot day is linked to poorer performance.
Dresser says the evidence suggests that heat can influence us in sometimes-indiscernible ways. "All of these [studies] seem to point to a reduced ability to think clearly and quickly and efficiently when the body is too hot," he says.
There's also research to suggest that heat can make you moodier or irritated, in part perhaps by raising cortisol levels and inducing a stress response.
Of course, you can acclimate to heat after several days of exposure, and our bodies have several built-in coping mechanisms that help us cool down. For instance, you'll begin to sweat sooner and blood flow to the skin increases, which can carry heat away from the body's core.
But given the extreme heat waves that are becoming more common, there's increasing interest in better understanding the mechanisms by which heat may exacerbate or set off mood and anxiety-related problems. Dresser points to a study published in JAMA Psychiatry in 2022 that found hospital ER visits for mental health conditions rise during extremely hot days.
"I think this is consistent with what a lot of physicians will tell you if they have worked during hot conditions," Dresser says. Mental health is a concern all of the time, "but it can become a bigger concern during really hot conditions," he says.
Multiple factors likely explain how heat exacerbates the risks, beyond changes in stress hormones and sleep disturbances. Dresser points out that there's an overlap between populations who are vulnerable to mental health issues and populations that are unhoused or have intermittent access to housing.
And, clearly, if someone is living outside during a heat wave, there's a greater likelihood of significant impact. "There may be complicated social issues going on," he says.
A better understanding of all of these factors could help inform strategies to prevent or manage the challenges. "As we learn to live in a warming world where the summers are getting hotter, we need to be extra alert to recognize when conditions are dangerous and take steps to stay safe," Dresser says.
One of the key strategies is to stay well-hydrated. This may sound obvious, but dehydration is common in the summer, and many people underestimate how much fluid they need to replace when they're sweating a lot or spending time outdoors.
In fact, the participants in the college dorm study benefited from staying well-hydrated. During the study, the researchers sent text messages asking all the participants how much liquid they'd consumed, and it turned out that the participants who slept in the hot dorm rooms and drank fewer than six glasses of liquid per day performed worse on the tests. And prior research has shown that being even a little dehydrated can impair cognitive performance.
It's a reminder that a simple step — remembering to drink plenty of water — can help protect not just our physical health, but our mental well-being, too.
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