You wake up on a hot morning. You're sweating, and you haven't slept well. Your head aches, the workday ahead makes you anxious, and the birds chirping outside make you want to scream.

If you lived through this summer's heatwaves in Europe, you know this is hardly an exaggeration of what a typical day can feel like. Record temperatures had Europeans feeling frustrated and dizzy. Part of the reason has to do with how our brain works.

Our brain makes up only 2% of our body mass, but eats up 20% of our energy. During a heatwave or short exposure to extremely high temperatures, the heart struggles to keep everything cool.

“The heart has to pump blood to the periphery of the body to help the body sweat, and for that it has to work hard. When heat exceeds our capacity to sweat, the supply of blood to the brain might not be optimal anymore, and the brain starts to overheat,” said Pieter Vancamp, a neurobiologist at the PhanLab in Nantes, France.

“You get this nauseous feeling, for instance, because the brain thinks that you're getting sick. The anxiety, the fatigue, you get it because your brain can't cope with the heat anymore and your brain cells communicate less well,” Vancamp said.

As a result, we struggle to focus, reason, understand and retain information. This applies not only to older people with pre-existing conditions, but to healthy, young people too.

“There is a rather broad temperature zone under which an average person can adapt. But at the edges, cognitive performance declines, especially on complex tasks which require a lot of attention,” said Mona Bielig, who researches environmental psychology at the University of Konstanz, where she works on a project tracking how heat affects our behaviour.

Those edges depend on humidity, personal sensitivity, exposure, and ability to adapt (like having AC or being close to a cool area). Some studies have tracked the decline to start around 30°C. This summer, many Europeans had to withstand temperatures well above 40°C.

At the same time, Bielig explained, “heat stress triggers a stress response. So there is a release of hormones such as cortisol, which then also leads to higher agitation and alertness.” Add poor sleep quality and dehydration to this overall discomfort, and you get tired, irritable, and impulsive.

“People make the mistake of thinking our bodies can adapt, and that we will get used to this heat by the next generation. But evolution takes much more time,” neurobiologist Vancamp said. “The only thing we can do is fight climate change and adapt our buildings, our clothing, and our technology to deal with the heat; that's what will make a difference, not relying on some sort of natural selection process.”

Writing this in 37°C heat was challenging, to say the least – it probably took me double the time it would have taken with a more bearable temperature. But hey, at least I'm not lazy or stupid: I'm just feeling the consequences of human-induced global heating.