Why does heat take such a toll on us, and what can we do about it?

Like all mammals, humans need a substantial amount of energy to function. The body produces this energy primarily by burning sugars – but the process also generates waste heat, which the body then has to get rid of.

There are several ways in which the body can release this excess heat. Within the normal ambient temperature range of around 10–23°C, roughly two-thirds of it is lost through thermal radiation. This is why the human body emits substantial infrared radiation – and how night-vision cameras, or indeed rattlesnakes, can detect us and other mammals.

If radiation alone is not enough to cool the body, it turns to another method: evaporation, or sweating. One of the more interesting evolutionary “tricks” of the human body is that we can sweat across almost our entire skin surface. This came at the cost of losing most of our body hair, but it gives us an advantage over most other animals – just think of dogs panting in the heat.

However, when humidity is high – as in the tropics or after a heavy summer downpour – or when the air is still and the moisture from evaporating sweat remains around the body, this mechanism also becomes less effective. This is where a fan can help: by bringing drier air into contact with the skin, it speeds up evaporation and helps cool the body.

The final option is to transfer heat to a medium that is cooler than body temperature – in practice, by sitting in a tub of cool water or wrapping ourselves in a cold, wet towel.

Why does heat take a greater toll on older people?

Several of the mechanisms described above depend on the body's own regulatory systems. As we age, however, thermoregulation becomes less precise, making it easier for the body to overheat and enter a dangerous temperature range. Older people also tend to sweat less readily and have a diminished sense of thirst. As a result, they are less able to cool themselves effectively through sweating and are also more prone to dehydration: blood plasma volume decreases, the blood becomes more concentrated, and the heart has to work harder.

For the body to release heat through the skin and through sweating, large amounts of blood need to flow to the skin, carrying heat from the body's core to its surface. When cooling is required, blood flow to the skin can increase from a resting level of a few hundred millilitres per minute to several litres per minute. The body redirects the blood needed for cooling away from the internal organs, causing blood pressure to fall and reducing blood flow to the brain.

At the same time, the heart has to work harder to pump the more concentrated blood, while the lungs and the kidneys, which help maintain the body's electrolyte balance, also come under greater strain. These effects may be further aggravated by medications commonly taken by older people, such as diuretics. In a frail person, the heart may no longer be able to cope with this additional burden, and the circulation can collapse.

As if that were not enough, the brain itself may receive too little blood and oxygen. This can lead to confusion, delirium and heatstroke, which can damage not only the brain but several other organs as well. This is why people in older age groups – particularly those with cardiovascular disease – need to take special care during periods of extreme heat.

Sleep requires cooling down, too

Higher-than-normal body temperature also takes a toll on healthy people, and it has a particularly strong effect on the quality of our sleep. Falling asleep requires the body's core temperature to drop by around 0.5–1°C. This means that heat at night is not simply uncomfortable – it actively interferes with one of the fundamental physiological conditions required for sleep.

The link between cooling and sleep is clearly observable, although its underlying cause has yet to be conclusively established. The scientific literature offers three possible explanations:

• The first hypothesis concerns the behaviour of neurons. The electrical activity of neurons and synaptic transmission are temperature-dependent: even a 1°C drop in temperature measurably reduces neuronal excitability and slows metabolism. This makes it easier for the neural networks that maintain wakefulness to “switch off”, while sleep-promoting neurons take over. Cooling, then, is not merely a consequence of sleep; it may actively help bring sleep about.
• The second explanation is energetic. The brain accounts for only around 2% of body mass, yet it is responsible for roughly 20% of the body's energy consumption at rest. Numerous regenerative processes take place during sleep, but overall energy demand falls because lower temperatures slow cellular metabolism. Although a night-time drop in body temperature of 0.5–1°C produces only a few per cent of energy savings in itself, even such a small advantage may have been significant over evolutionary timescales.
• A third, particularly intriguing hypothesis is linked to the function of sleep itself. During deep sleep, the extracellular space in the brain expands and the activity of the so-called glymphatic system increases. This system helps remove proteins and other metabolic waste products that accumulate during the day, including beta-amyloid. It is possible that slightly lower temperatures favour these regenerative processes and help reduce oxidative damage. There is some indirect evidence for this idea, but it cannot yet be regarded as proven.

Evolutionary advantages?

It is also worth considering the question from an evolutionary perspective. The alternation of day and night, and of active and resting periods, has been an important part of survival since the dawn of life. Circadian changes in activity occur even in single-celled organisms and organisms without a nervous system, although these are not usually described as “sleep”. Sleep-like states have even been observed in cnidarians such as jellyfish and hydras, despite their extremely simple nervous systems. In reptiles, sleep already separates into two familiar phases: REM sleep and non-REM, or slow-wave, sleep.

Rest may be even more important for warm-blooded animals. An animal that maintains a constant body temperature may be five to ten times more active than a reptile, but it also requires five to ten times more energy – energy that has to be obtained by finding and consuming substantially more food. This is probably why, very early in evolutionary history, organisms developed the ability to alter their metabolism during periods of rest – a change that, in larger animals, is also accompanied by a fall in body temperature.

Over the course of evolution, many essential processes came to take place during sleep. These include growth and regeneration, the restoration of the brain's internal balance and the consolidation of memories, which is accompanied by dreaming. If we are unable to cool down sufficiently, the activity of the brain's arousal systems does not decrease as much as it should. This makes it harder to fall asleep and causes sleep to remain more superficial. The processes needed for brain regeneration do not get properly under way, and things we have learned are not transferred into long-term memory as effectively. This is why we become irritable and struggle to function properly after a poor night's sleep.

What can we do about the heat – and to sleep better?

• Drink plenty of fluids. Because sweating causes us to lose not only water but mineral salts as well, these need to be replaced. The most important is sodium, which is found in table salt, although during extreme physical exertion magnesium, calcium and potassium also need to be replenished.
• Cool yourself with a fan or a wet towel.
• Encouraging blood flow to the extremities can help lower core body temperature. This is why washing your hands and feet in warm water before bed – or even wearing socks while you sleep – can make it easier to fall asleep.

You can read the two blog posts (in Hungarian) on which this article is based here and here.