Memory, Attention, and Mood: How Heat Waves Affect the Brain

Climate change is increasingly worrying citizens. And not only during extreme heat waves. Detrimental effects have been observed on the brain and appear at temperatures still considered “normal” for a large portion of the population. This is what a survey by the Human Adaptation Institute reveals, reporting unseen consequences of heat on our mental and cognitive health.

Heat Is Transforming Our Environment and Our Brains

Europe has faced increasingly frequent and intense heat events. After the historic 2003 heat wave, which caused tens of thousands of deaths in France and other European countries, heat waves are multiplying under the influence of anthropogenic climate warming. This global climate shift affects humanity on several levels. According to the World Health Organization, heat is now a major public health risk, with heat-related mortality rising sharply in recent years.

But beyond acute mortality, it is the heat itself that affects our brain, according to the study “Humans Facing Heat” presented by Christian Clot, president of the Human Adaptation Institute. The researchers interviewed more than 4,000 people and analyzed these responses alongside physiological and meteorological data. Christian Clot notes, in remarks reported by Futura Sciences, that “we did not expect to observe such a rapid onset of psychological and mental degradation. From an average of 24°C over 24 hours or 32°C at peak,” effects can be felt on mental functions.

Heat Impairs Cognition and Mental Functions

The impact of heat on the brain is measurable at several levels. Independent scientific studies show that high ambient temperatures can reduce cognitive performance even in the absence of serious illness. A 2018 study revealed that during a heat wave, adults living without air conditioning exhibited significant cognitive deficits compared with a group equipped with air conditioning, with slower reaction times and reduced information-processing abilities.

Recent work conducted in preadolescent children also suggests that exposure to high temperatures is associated with reduced connectivity in several core brain networks (memory, attention, decision-making), which could affect how the brain communicates internally. This disruption of brain networks is consistent with other observations: higher temperatures are linked to lower cognitive performance, impaired attention, and poorer memory, even when temperatures are not extreme.

How Heat Disrupts the Brain

The biological mechanisms by which heat affects the brain are multiple. Dehydration, which often accompanies intense heat, can alter cerebral blood flow and affect connections between neurons, impairing cognitive function and judgment. Another likely pathway is the impact on sleep quality. The body’s thermal regulation during the night is essential for restorative rest.

When ambient temperatures stay high, as is increasingly common with tropical nights during heat waves, sleep is disturbed, which can indirectly degrade cognitive abilities and mood. Moreover, on a broader neurological level, several conditions such as multiple sclerosis show a marked worsening of symptoms under heat, a well-documented neurological phenomenon known as Uhthoff’s phenomenon.