We don't just sweat: extreme heat affects sleep, mood and concentration.

Every summer, the same argument arises: that it has always been hot and that the current figures lack historical context . However, June 2025 has been crowned as the warmest in Spain since records began (since 1961), with maximum temperatures more than 4°C above normal levels. According to scientific estimates, those born in 2020 could face twice as many extreme weather events as those born six decades earlier. Heat waves will also be more frequent and longer in many parts of the world.

An example of the latter is the second heat wave of this summer, which began last Sunday, August 3, and is initially expected to last until Sunday, August 10. However, after extending for three days, it is not known for certain whether this wave could continue for more days, which would have a serious impact on the health of the population. The summer of 2023 left nearly 70,000 deaths related to high temperatures in Europe alone , and the World Health Organization (WHO) warns that heat-related mortality has increased by 85% since the beginning of the century.

Beyond the numbers, it's important to understand how the human body gets rid of excess heat and how it affects our physical and mental health. To shed light on these issues, the Science Media Centre (SMC) Spain has offered some insights.

Although we often say colloquially that we're "hot," in physical terms that's not entirely correct . Heat is energy in motion: it's the transfer of energy between two bodies at different temperatures. In other words, it only exists during that journey: you feel hot, but you don't actually have it. What you have is a high internal energy, which is what temperature measures.

If you're lucky or have air conditioning , you may be losing a good part of your heat as you read this in the form of radiation , which is the emission of heat in the form of electromagnetic waves, always from a warmer entity to a cooler one. In this way, we lose up to 65% in mild conditions, but its significance is minimal at high temperatures. You can also lose some heat through conduction, for example, when holding a very cold glass, but this isn't the ideal technique either.

Ultimately , the human body's secret weapon for reducing heat is evaporation: the process by which sweat turns into vapor and extracts energy from the body. In hot environments and during exercise, this system can account for up to 85% of heat loss.

The human body activates a complex network of physiological responses to heat. Thermoreceptors distributed throughout the body send signals to the hypothalamus, the center that regulates body temperature. From there, it orders increased sweating, dilation of blood vessels in the skin, increased heart rate, and suppressed thyroid activity to prevent further heat generation.

In addition, the body seeks to conserve fluids: it stimulates the sensation of thirst, releases antidiuretic hormone to encourage the kidneys to retain water, and activates mechanisms to retain sodium, which helps prevent dehydration. All of this forms a fairly effective, though not unlimited, survival orchestra.

But heat doesn't just affect the body, it also affects the brain. Beyond heat exhaustion, high temperatures cause irritability, lack of concentration, and a general feeling of malaise . Recent meta-analyses have shown an association between heat and an increase in visits for mental health issues . Academic performance has also been observed to decline during prolonged periods of heat, especially in areas with less access to air conditioning, as reflected in a study in Nature Human Behavior based on data from 12,000 U.S. school districts.

In the workplace, both physical and cognitive productivity are affected . Neuroimaging studies show that the brain redistributes blood flow: it increases in regulatory areas like the hypothalamus and decreases in others related to attention and emotions , which can lead to nervousness, distraction, and a reduced ability to perform complex tasks.

Much of the fatigue associated with heat is related to sleep. Sleeping well requires a drop in body temperature , something that ambient heat makes difficult. In fact, research on pre-industrial societies reveals that they didn't go to bed at dusk, but rather hours later, when the outside temperature dropped. Deep sleep suffers when this temperature drop isn't achieved, which increases the feeling of daytime exhaustion.

Yes, the body can acclimatize to heat . It does this by optimizing its heat dissipation systems: it begins to sweat earlier and more, but with a lower salt concentration; it improves blood flow efficiency; it fine-tunes kidney regulation; and it even increases the production of protective proteins.

In athletes, these changes can be completed in about two weeks of progressive heat exposure, especially with moderate exercise. However, this recommendation does not apply to the general population, as physical activity in extreme conditions carries risks. Some research suggests that sauna sessions or hot baths could be useful in facilitating acclimatization in older adults, although there is still no scientific consensus. However, adaptation is temporary: it begins to fade after a week without exposure and can disappear completely after a month.