Do neurons melt? How extreme temperatures affect the functioning of the brain

The first heat wave of the summer has placed eight out of 10 Spanish municipalities under warning for risks to human health. Although the effects of dehydration on the body is the main concern, the brain also suffers from this upward trend in temperatures: 2023 set records as the hottest year on the planet since records began in the mid-19th century. Recent studies have shown that excessive heat reduces cognitive abilities, both for studying and working. In addition, while the brain works hard to keep the body cool, extreme temperatures increase aggressiveness and stress and particularly affect patients with certain psychiatric disorders.

The brain is a temperature-sensitive organ that is not equipped to function properly at 45ºC. At such high temperatures cognitive function slows down, as Sandra Giménez, a clinical neurophysiologist at the Hospital de la Santa Creu i Sant Pau in Barcelona, explains: “Extreme heat affects all the cognitive functions of the brain: our ability to react, our response capacity, memory, etc. Everything becomes much harder; we go much slower. We are not going to say that neurons melt, but there is an effect. Performance is much poorer in high temperatures.”

Scientific evidence supports this. Taking a test on a day when the temperature is higher than 32ºC results in a 14% reduction in grade score relative to taking that same test at 22ºC and reduces the odds of passing a subject by nearly 11%, according to a 2018 study carried out in New York public schools. “I estimate that, during the period 1998-2011, more than 510,000 exams that would otherwise have been passed were failed due to high temperatures, affecting at least 90,000 students, possibly many more,” concludes Jisung Park, a professor at the Harvard Kennedy School and author of the study.

Other research, also conducted in the U.S., noted that “the rate of learning decreases with an increase in the number of hot school days.” Another study, which compared the performance of Boston University students during a 2016 heat wave, concluded that those living in rooms without air conditioning (at an average temperature of 27ºC) displayed a 13% slower reaction time on arithmetic tests and got almost 10% fewer correct answers per minute than their peers who had air conditioning (at an average of 22ºC).

While most of these studies have been conducted in academic settings, the cognitive impairment caused by extreme heat also affects the workplace: research conducted in 2006 found that the highest productivity is achieved at a temperature of around 22ºC. At eight degrees warmer, performance levels were reduced by almost 9%.

“There are numerous studies that establish links with mental health, mood, and brain behavior with heat, so people with mental health problems are particularly vulnerable,” says meteorologist and science communicator Mar Gómez, who notes there is research showing that higher temperatures decrease positive emotions such as joy or happiness and increase negative ones such as anger or stress.

More aggressiveness and worse mental health

“We know that people with schizophrenia can experience difficulties with body temperature regulation and that temperature changes can alter symptoms of mood disorders. Additionally, some psychiatric medications, including certain antidepressants and antipsychotics, can affect the way the body regulates temperature and people taking them are especially vulnerable to the effects of extreme heat,” Gómez explains.

Among the negative emotions associated with heat, anger is one of the most studied. So are two of its direct consequences: aggressiveness and violence. “Extreme heat can increase irritability and decrease self-control, which can translate into more aggressive behavior. The relationship between intense heat and aggressiveness is real,” says Valentín Martínez, who holds a PhD in psychology from the Complutense University of Madrid and is a member of the Madrid College of Psychology.

A study published in 2022 in The Lancet that analyzed four billion tweets concluded that very high or very low temperatures aggravate aggressive online trends and increase hate speech. The increase in such tweets was 22% on days with extreme temperatures (of 42ºC to 45ºC). Another study found a direct linear increase in car horn use with increasing temperature. There are even studies that have concluded that each one-degree increase in annual temperatures would be associated with an average increase of nearly 6% in the number of homicides.

An investigation led by experts in gender violence, specialists in epidemiology and by police and Civil Guard psychologists who analyzed the months from May to September in the period 2008-2016 in the Madrid region concluded that for every degree in which the maximum daily temperature exceeds the threshold of 34ºC, femicides within relationships increase by 28.8% compared to the average. “This does not mean that the Madrid study revealed that gender violence is a direct consequence of heat. Far from it. Its conclusion was that heat is a factor that influences the increase in violence, along with other causes,” clarifies Gómez. This opinion is corroborated by Giménez, who believes that high temperatures can make anyone more aggressive: “It doesn’t mean that we’re all going to start stabbing people. There has to be a psychopathological basis.”

How heat impacts the brain

The explanation for all these consequences, according to Valentín Martínez, could be found in the fact that “heat forces the brain to work harder to regulate body temperature, which negatively affects mental capacity,” as the brain allocates a large part of its resources to keeping the body cool.

“We have to be aware that our brain functions properly, among other things, thanks to the hypothalamus, which is the coordinator of the autonomic nervous system and acts as a kind of internal thermometer of the brain. When it detects that there are changes between its own temperature and that of the thermoreceptors in the skin, the hypothalamus establishes the mechanisms to regulate it,” explains Gómez.

These mechanisms are sweating, vasodilation, or adrenaline production. According to the expert, adrenaline production “is one of the causes of greater irritability when we are subjected to periods of intense heat.”

In addition to this overexertion of the brain, there is another extremely important factor: sleep. “On tropical nights, when the ambient temperature does not drop below 20ºC, our brain is overexcited and body sweating increases, so that our body is in a state similar to that of having to perform intense physical activity, which is totally incompatible with rest or comfortably maintaining sleep,” the meteorologist points out. “It’s a fish that bites its own tail,” adds Giménez, who is the coordinator of the cognition and sleep working group at the Spanish Sleep Society (SES). According to this expert, excessive heat causes a kind of vicious circle. We sleep worse, which makes us cognitively slower, more anxious, and more irritable; and the heat during the day then accentuates these symptoms. “Control is lost at the prefrontal level of the brain and the brake on the amygdala, which is the area that processes emotions, decreases so that everything negative is magnified,” she explains.

There is no magic wand to counteract these effects. The advice, Martínez points out, is common sense: stay well hydrated and drink enough water; avoid prolonged exposure to extreme heat, especially during the central hours of the day; look for cool, air-conditioned places; wear light, light-colored clothing to facilitate perspiration; limit intense physical activity outdoors during the hottest hours; eat cool, light, water-rich foods such as fruits and vegetables; and do everything possible to get enough rest.

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