Feeling slow, distracted, unable to finish an email during a heatwave is very common. During episodes of
extreme heatmany describe a real
brain fog. Neuroscientists who study these heat waves show that it is not an impression: our brain then devotes a large part of its energy to cooling us down.
Concretely, the heat solicits both the circuits of
thermoregulationfluid balance and sleep. By comparing the work summarized in The Conversation, the Brain Research Foundation and Health Insurance, we see that our attention, memory and decision-making abilities diminish well before heat stroke.
When extreme heat commandeers the brain to cool you down
On the surface of the skin, thermoreceptors
send signals to thehypothalamusthe “thermostat center” of the brain responsible for keeping a stable temperature. As Dan Baumgardt, lecturer at the University of Bristol, explains in The Conversationthis center triggers cutaneous vasodilation, perspiration and redistribution of blood to the skin. This continuous management mobilizes resources that the brain can no longer devote entirely to thinking.
When the muscles also produce heat, for example during physical exertion, this load increases further. The Brain Research Foundation reports a study where participants made more errors and were less productive at 35°C than at 20°C. Resources normally dedicated to complex tasks are then redirected towards a single objective: avoiding overheating.
Neurons, dehydration and electrolytes: the brain in slow motion
THE
neurons work through enzymes, proteins and the passage of ions that trigger the release of neurotransmitters. The Conversation describes processes that are very sensitive to temperature: when it is hot, they become less precise, which impairs working memory, concentration and decision-making. The brain contains approximately 78% water, making it very vulnerable to dehydration.
When sweating intensifies, a risk of heat exhaustion may appear, with body temperature rising to around 38 to 40°C, fatigue and headaches. If nothing is done, a
heat stroke may occur, with temperature equal to or greater than 40°C and behavioral or consciousness disorders. Cerebral blood flow drops, electrolytes become imbalanced, which slows attention and reaction times.
Heatwave, sleep and brain fog: how to limit the damage
The heat also weighs on the night. It weakens sleep, reducing the duration of deep sleep which restores our cognitive functions. “When it’s very hot, your brain also takes on the stress“, explains Dr. Gérald Kierzek, emergency physician and medical director of True Medical. Broken nights, high cortisol, irritability: when you wake up, the brain is already dealing with fatigue and residual heat.
Experts’ recommendations converge: stay in cool rooms, limit efforts to cooler hours and drink regularly before feeling thirsty. Simple actions can add a little extra: light sheets, lukewarm showers or cooling the neck and forearms. If confusion, incoherent speech or intense headaches appear, you must cool down, hydrate quickly and call for help.