The world is getting hotter – this is what it is doing to our brains

BBC; Theres Lüthi; 1 August 2025;

As heatwaves become more intense with climate change, scientists are racing to understand how extreme heat changes the way our brains work.

When Jake was five months old, he had his first tonic-clonic seizure, his little body stiffening and then jerking rapidly. “It was extremely hot, he had overheated and we witnessed what we thought would be the scariest thing we would ever see,” says his mother, Stephanie Smith. “Unfortunately, it wasn’t.”

Seizures began to crop up often in hot weather. As soon as the stifling, humid days of summer would arrive, the family would resort to all kinds of cooling methods and a fierce battle to keep the seizures at bay would ensue.

Following a genetic test at the age of 18 months, Jake was diagnosed with Dravet Syndrome, a neurological condition that includes a form of epilepsy and affects around one in 15,000 children. Seizures are often accompanied by intellectual disability and a range of comorbidities such as autism and ADHD, as well as difficulties with speech, mobility, eating and sleep. Heat and sudden temperature changes can bring on a seizure.

Jake is now 13 years old, but has endured countless seizures with the turn of the weather, his mother says. “Increasingly hot summers and heatwaves are adding to the burden of living with this already devastating condition,” says Smith.

Dravet Syndrome is just one of many neurological diseases that are exacerbated by higher temperatures, says Sanjay Sisodiya of University College London and a pioneer in the field of climate change’s impact on the brain. A neurologist who specialises in epilepsy, he frequently heard from patients’ families that they had more troubles during heatwaves. “And I thought to myself, of course, why shouldn’t climate change also affect the brain? After all, so many processes in the brain are involved in how the body copes with heat.”

As he dug into the scientific literature, he discovered a range of neurological conditions that are made worse by rising heat and humidity, including epilepsy, stroke, encephalitis, multiple sclerosis, migraine, along with a number of others. He also discovered that the effects of climate change on our brains are already becoming visible.

During the 2003 European heatwave, for example, about 7% of the excess deaths involved direct neurological problems. Similar figures were also seen during the 2022 UK heatwave.

But heat can also alter other ways our brains function – making us more violent, grumpy and depressed. (Find out more about how heatwaves warp the mind in this article by Zaria Gorvett.)

So, as the world warms due to climate change, what can we expect the effect on our brains to be?

The human brain is, on average, rarely more than 1C (1.8F) higher, on average, than our core body temperature. Yet our brains – as one of the more energy-hungry organs in our bodies – produce a fair amount of their own heat when we think, remember and respond to the world around us. This means our bodies have to work hard to keep it cool. Blood circulating through a network of blood vessels helps to maintain its temperature, whisking away excess heat.

This is necessary because our brain cells are also extremely heat sensitive. And the function of some of the molecules that pass messages between them are also thought to be temperature dependent, meaning they stop working efficiently if our brains get too hot or too cold.

“We don’t fully understand how the different elements of this complicated picture are affected,” says Sisodiya. “But we can imagine it like a clock, where all the components are no longer working together properly.”

Although extreme heat alters how everyone’s brains work – it can, for example, adversely affect decision making and lead to people taking greater risks – those with neurological conditions are often the most severely affected. This is for many reasons. For example, in some diseases, perspiration may be impaired.

“Thermoregulation is a brain function and can be disrupted, if certain parts of the brain are not functioning properly,” says Sisodiya. In some forms of multiple sclerosis, for instance, the core body temperature appears to be altered. In addition, some drugs that treat neurological and psychiatric conditions such as schizophrenia affect temperature regulation, leaving those taking them more vulnerable to heatstroke, or hyperthermia as it is known medically, and at a greater risk of heat-related death.

Have you noticed any cognitive or mood swings when it’s too hot outside? You’re not alone.

Heatwaves – and elevated nighttime temperatures especially – can affect people’s sleep, affecting our mood and potentially worsening the symptoms of some conditions. “For many people with epilepsy, poor sleep can increase the risk of having seizures,” says Sisodiya.

Evidence suggests that hospital admissions and mortality rates among people with dementia also increase during heatwaves. Part of this may be due to age – older people are less able to regulate their body temperature – but their cognitive impairment may also mean they are less able to adapt to extreme heat. They may not drink enough, for example, or forget to close the windows, or go out into the heat when they shouldn’t.

Rising temperatures have also been linked to an increase in stroke incidents and mortality. In one study that analysed stroke mortality data from 25 countries, researchers found that out of 1,000 deaths from ischemic stroke, the hottest days contributed two excess deaths. “That may not seem like a lot,” says Bethan Davies, a geriatrician at University Hospitals Sussex, in the UK. “But given that there are seven million deaths from strokes a year worldwide, heat may well be contributing to over 10,000 additional stroke deaths per year.” She and her co-authors warned that climate change is likely to exacerbate this in years to come.

A disproportionate share of the burden of heat-related stroke will be in middle- and low-income countries, which are already most affected by climate change and experience the highest rates of stroke. “Rising temperatures will exacerbate health inequalities both between and within countries and social groups,” says Davies. A growing body of evidence suggests that older people as well as those with a low socioeconomic position are at an increased risk for heat-related mortality.

A hotter world is also harming the neurodevelopment of the very youngest. “There is a link between extreme heat and bad pregnancy outcomes such as premature births,” says Jane Hirst, professor of global women’s health at Imperial College London in the UK. One recent systematic review of the scientific research found that heatwaves are associated with a 26% increase in preterm births, which can lead to neurodevelopmental delays and cognitive impairments.

“However, there is a lot we do not know,” adds Hirst. “Who is most vulnerable and why? Because clearly, there are 130 million women who have babies every year, a lot of them in hot countries, and this does not happen to them.”

What we’re seeing today in people with neurological disorders could become relevant for people without neurological disorders as climate change progresses – Sanjay Sisodiya

Excessive heat due to climate change may also put additional strain on the brain, leaving it more vulnerable to damage that can lead to neurodegenerative diseases. Heat also affects the barrier that normally protects the brain, making it more permeable and increasing the risk that toxins, bacteria and viruses can cross over into our brain tissue.

This could become more important as temperatures increase, as so too will the spread of mosquitos that transmit viruses that can cause neurological disease, such as Zika, chikungunya and dengue. “The Zika virus can affect foetuses and cause microcephaly,” says Tobias Suter, a medical entomologist at the Swiss Tropical and Public Health Institute. “Rising temperatures and milder winters mean that the mosquito breeding season begins earlier in the year and ends later.” (Read David Cox’s story on how the US’s mosquito season is already changing.)

Heatwaves are capable of influencing a whole range of factors, from the electrical firings of the nerve cells to suicide risk, climate anxiety and even the stability of medication for neurological conditions.

But exactly how rising temperatures affect our brains are still being investigated by scientists. Heat affects people in very different ways – some thrive in hot weather, others find it unbearable. “Different factors might be relevant for this differential sensitivity, and one of them may be genetic susceptibility,” says Sisodiya. Genetic variants could influence the structures of proteins that might render some people more vulnerable to the impacts of climate change.

“There may be thermo-latent phenotypes that will only become apparent when those environmental pressures are sufficient to bring them out,” he says. “What we’re seeing today in people with neurological disorders could become relevant for people without neurological disorders as climate change progresses.”

There are still other questions that remain to be answered too. For example, is it the maximum temperature, is it the length of a heatwave or the nighttime temperature that has the greatest impact? It may well differ for each person or by neurological condition.

But identifying who is at risk and why will be crucial to developing strategies to protect the most vulnerable. These could include early warning systems or insurance to compensate day labourers for lost wages due to extreme heat.

“The era of global warming has ended, the era of global boiling has arrived,” UN Secretary-General Antonio Guterres announced, when July 2023 was confirmed to be the hottest month on record. Climate change is here and it is intensifying. The era of the hot brain is just beginning.