Heat Disrupts the Brain: Scientists Are Starting to Understand Why

The heat in Western Europe has broken June records: thermometers in the UK hit 36.1 °C, the highest June reading in the entire history of meteorological observation. The apparent temperature reached 39 °C. The dangerous heatwave has swept across several countries on the continent at once. But its consequences go far beyond heatstroke and dehydration: neuroscientists and physicians are documenting a notable and still poorly understood effect of extreme heat on brain function, and they are beginning to look for answers systematically.

What heat does to cognitive function

During heatwaves, people concentrate worse, react more slowly and make more mistakes when making decisions. One of the first rigorous experiments of this kind was carried out at Harvard in 2018: students living in hot rooms without air conditioning during a heatwave in Boston had reaction speeds 13% lower than their peers in air-conditioned dorms, with the same academic workload.

The key mechanism of impairment is disrupted sleep. When nighttime temperatures do not fall below 25 °C, the body cannot cool down to the level needed for full recovery. Slow-wave, or deep, sleep suffers most: it is during this stage that the brain clears metabolic waste and transfers information from short-term to long-term memory. After three or four nights without good sleep, working memory, the ability to concentrate and the capacity for sound decision-making deteriorate noticeably.

Dehydration is an additional factor: losing even 1–2% of the body's fluid reduces attention and worsens short-term memory.

Heat and mental health

Cognitive effects are only part of the picture. The mental health data is no less worrying:

• During heatwaves, the number of emergency hospitalizations for psychotic episodes rises
• Anxiety disorders and depression statistically worsen under chronic overheating
• Several large epidemiological studies have found a link between rising average temperatures and suicide rates
• In extreme heat, crime statistics show more cases of aggression and violence
• The most vulnerable are the elderly, patients with chronic illnesses and those without access to air conditioning

Most of this data, however, is correlational. The direct mechanisms through which heat changes the psyche have yet to be established.

Why there are no answers yet

The main difficulty is that the effects are multifactorial. Heat affects the brain through several channels at once: body temperature rises, the body loses water and electrolytes, sleep is disrupted, blood supply to the brain changes and the inflammatory response intensifies. Separating these factors in real-world conditions is extremely hard, and laboratory experiments cannot fully reproduce multi-day extreme heat.

Another problem is geographic gaps. Most large studies were conducted in temperate climates: the US, Europe and Japan. Regions that chronically suffer from extreme heat, such as South Asia, West Africa and the Persian Gulf, are far less studied. It is unknown whether the brain adapts to constantly high temperatures, or whether that adaptation has a limit.

What this means

Heatwaves are getting longer and more intense, and they cover ever larger areas. The cognitive and mental effects of heat are no longer a peripheral topic but a medical and social problem in their own right. Once scientists understand the specific mechanisms, it will change recommendations for employers, urban planners and health systems, especially in countries where air conditioning is still a luxury.