Illustration by Edwin Fotheringham

In his meticulous diaries, written from 1846 to 1882, the Harvard librarian John Langdon Sibley complains often about the withering summer heat: “The heat wilts & enervates me & makes me sick,” he wrote in 1852. Sibley lived before the age of air-conditioning, but recent research suggests that his observation is still accurate: summer really does tend to be a time of reduced productivity. Our brains do, figuratively, wilt.

One of the key issues is motivation: when the weather is unpleasant, no one wants to go outside, but when the sun is shining, the air is warm, and the sky is blue, leisure calls. A 2008 study using data from the American Time Use Survey found that, on rainy days, men spent, on average, thirty more minutes at work than they did on comparatively sunny days. In 2012, a group of researchers from Harvard University and the University of North Carolina at Chapel Hill conducted a field study of Japanese bank workers and found a similar pattern: bad weather made workers more productive, as measured by the time it took them to complete assigned tasks in a loan-application process.

When the weather improved, in contrast, productivity fell. To determine why this was the case, the researchers assigned Harvard students data entry on either sunny or rainy days. The students were randomly assigned to one of two conditions: before starting to work, they were either shown six photographs of outdoor activities in nice weather, such as sailing or eating outdoors, or were asked to describe their daily routines. The researchers found that participants were less productive when they’d viewed pleasant outdoor photographs. Instead of focussing on their work, they focussed on what they’d rather be doing—whether or not it was actually sunny or rainy outside (though the effect was stronger on sunny days). The mere thought of pleasant alternatives made people concentrate less.

But each season has its share of attractive days—and a skier’s mind would likely have many opportunities to wander in the dead of winter. There’s evidence, however, that in summer, our thinking itself may simply become lazier. In 1994, Gerald Clore, a pioneer in researching how ambient mood-altering phenomena affect cognition and judgment, found that pleasant weather can often lead to a disconcerting lapse in thoughtfulness. Clore’s team approached a hundred and twenty-two undergraduates on days with either good or bad weather and asked them to participate in a survey on higher education. The better the weather, the easier it was to get the students to buy into a less-than-solid argument: on days that were sunny, clear, and warm, people were equally persuaded by both strong and weak arguments in favor of end-of-year comprehensive exams. When the weather was rainy, cloudy, and cold, their critical faculties improved: in that condition, only the strong argument was persuasive. Clore and his colleagues concluded that pleasant weather led people to embrace more heuristic-based thinking—that is, they relied heavily on mental shortcuts at the expense of actual analysis.

Summer weather—especially the muggy kind—may also reduce both our attention and our energy levels. In one study, high humidity lowered concentration and increased sleepiness among participants. The weather also hurt their ability to think critically: the hotter it got, the less likely they were to question what they were told.

The shift toward mindlessness may be rooted in our emotions. One common finding is a link between relative sunshine and happiness: although people who live in sunnier places, like Southern California, are no happier than those who live in the harsher conditions of the Midwest, day-to-day variations in sunshine make a difference. People get happier as days get longer and warmer in the approach to the summer solstice, and less happy as days get colder and shorter. They also report higher life satisfaction on relatively pleasant days. The happiest season, then, is summer.

A good mood, generally speaking, has in turn been linked to the same type of heuristic, relatively mindless thinking that Clore observed in his pleasant-weather participants. On the flip side, a bad mood tends to stimulate more rigorous analytical thought. Weather-related mood effects can thus play out in our real-life decisions—even weighty ones. In one recent project, the psychologist Uri Simonsohn found that students were more likely to enroll in a university that was famous for its academic rigor if they visited on days that were cloudy. When the weather turned sour, he concluded, the value they placed on academics increased.

There’s a limit, however, to heat’s ability to boost our mood: when temperatures reach the kind of summer highs that mark heat waves all over the world, the effect rapidly deteriorates. In a 2013 study of perceived well-being, the economist Marie Connolly found that on days when the temperature rose above ninety degrees, the negative impact on happiness levels was greater than the consequences of being widowed or divorced.

Conversely, the effects of heat on our brains aren’t entirely negative. Many of the behaviors that psychologists study follow a so-called inverted-U pattern: as one factor steadily increases, a related behavior improves, plateaus, and then starts to deteriorate. A famous example of this is the Yerkes-Dodson curve, which charts the effect of stress on how well someone performs a given task. If we experience too little stress, or too much, our performance suffers. Like Goldilocks, we want to get it just right. Similarly, our cognitive abilities seem to improve up to a certain temperature, and then, as the temperature continues to rise, quickly diminish. An early study suggested that the optimal temperature hovered around seventy-two degrees Fahrenheit. A more recent review of the literature shows a target of twenty-seven degrees Celsius, or roughly eighty-one degrees Fahrenheit. (An important caveat, however, is that neither of these studies take humidity or sunshine into account, two major factors when it comes to assessing the influence of summer weather on behavior.)

Maybe best of all, blistering heat does give us a perfectly good reason to eat ice cream: studies have shown again and again that blood glucose levels are tied to cognitive performance and willpower. A bite of something frozen and sweet, boosting depleted glucose stores, might be just what a brain needs as the temperature spikes.

Illustration by Edwin Fotheringham.