What does heat do to our body? How circulation, brain and mood react

Why do we suddenly feel so drained in the heat? Why is it harder to concentrate, even though we are just sitting at a desk? And why do we get irritable more quickly after several hot nights?

There are real physical reasons behind this. Our body constantly tries to keep its core temperature within a narrow range. When it gets hot outside, heat release has to work harder. Blood vessels in the skin widen, we sweat more and lose fluid in the process. At the same time, high temperatures can affect sleep, concentration and wellbeing.

Heat is therefore much more than an unpleasant feeling on the skin.

In short: what does heat do to the body?

In heat, the body widens blood vessels in the skin and produces more sweat to release heat. This causes water and electrolytes to be lost, and the circulation is put under more strain. Fatigue, thirst, headaches, dizziness and concentration problems can be the result.

The brain reacts too: studies show that high temperatures can impair cognitive performance. Warm nights also make sleep harder, which can further increase fatigue, concentration problems and irritability the next day.

1. First, your body tries to get rid of the heat

The human body constantly produces heat itself. Normally it can release this without any problem into its surroundings. As the outside temperature rises, this becomes harder.

Two mechanisms then become especially important:

Blood flow to the skin increases. Blood vessels close to the body's surface widen. This allows more heat to be carried from inside the body to the skin and released into the surroundings.

You sweat. When sweat evaporates on the skin, this process draws heat from the body and has a cooling effect.

In high humidity, evaporation works less well. That is why a muggy day can feel more demanding than dry heat at the same temperature.

The Federal Institute of Public Health (BIÖG) explains the health effects of heat in detail and names, among other things, fluid loss, strain on the circulation, fatigue and concentration problems as possible consequences.

2. Why does heat make us so tired?

“Why does heat make you tired?” is one of the most common questions around high temperatures. The answer is not a single mechanism.

Several things come together:

• Blood vessels in the skin widen.
• Blood pressure can drop as a result.
• Through sweating, the body loses fluid.
• Circulating blood volume can decrease.
• Heart rate can rise to keep up supply.
• Physical movement becomes more strenuous in heat.
• Hot nights can make sleep worse.

As heat strain increases, the BIÖG describes, among other things, weakness, exhaustion, low blood pressure and an increased heart rate.

So you are not imagining that summer tiredness. A normal working day, the walk to the train or a stroll through the city can genuinely feel more exhausting in great heat.

If you are especially interested in why we feel so exhausted on hot days, you can also find our guide Heat stress in everyday life: why heat makes you tired.

3. Sweating costs mainly fluid and minerals

Sweating makes sense. Without this mechanism we would be much worse at releasing excess body heat at high temperatures.

In the process, however, the body loses water and electrolytes such as sodium. The more intensely and the longer you sweat, the more important it becomes to replace these losses.

Fluid deficiency can show up, among other things, as:

• thirst
• dry mouth
• headaches
• fatigue
• dizziness
• reduced performance
• concentration problems

With more severe fluid deficiency, confusion and pronounced weakness can also occur.

That is why the BIÖG recommends drinking regularly on hot days, preferably water or unsweetened drinks. Anyone who has to follow a medically prescribed fluid intake due to a heart or kidney condition should not change this on their own.

4. What does heat do to our brain?

Even young and healthy people can think less clearly in the heat.

A widely noted study from the Harvard T.H. Chan School of Public Health examined 44 young adults during a heatwave. Students living in non-air-conditioned buildings reacted more slowly in two cognitive tests during the heat than the comparison group in air-conditioned rooms. Attention, processing speed and working memory were among the aspects studied.

That does not mean everyone suddenly becomes “dumber” at 30 degrees. Cognitive performance depends on many factors, and not every task is affected equally.

But research shows: heat can measurably affect attention, reaction speed and more complex thought processes.

In everyday life this can feel like:

• you read the same sentence twice
• decisions feel harder
• you get distracted more easily
• tasks feel more tiring
• you react more slowly
• motivation drops

On top of that come indirect effects. Anyone who has drunk too little or slept badly starts the day with worse conditions anyway.

5. Why can we be more irritable in the heat?

Heat does not only affect the body and concentration. Our psychological wellbeing can suffer too.

Scientific studies find links between high ambient temperatures and worse mood, increased irritability and greater psychological strain. A systematic review on ambient temperature and mental health summarises numerous studies on this link.

Context matters here: this does not mean a hot summer day automatically causes a mental illness.

In everyday life, several strains probably combine:

• physical discomfort
• poor sleep
• fluid loss
• less recovery
• warm homes
• noise and crowded transport
• restricted movement
• extra physical strain

It is therefore not particularly surprising that you react less calmly at 34 degrees on a crowded train than on a mild spring day.

For people with existing mental health conditions, however, extreme heat can be more relevant. Studies and reviews show, at a population level, a link between high temperatures and an increase in certain mental health problems.

6. Why do we sleep worse in the heat?

For us to fall asleep well, our body temperature also changes. In the evening and at night, the body needs to be able to release heat.

If the bedroom stays very warm, this process becomes harder.

A large study with more than seven million recorded nights from over 47,000 people across 68 countries showed that rising night-time temperatures were linked to shorter sleep duration. An important effect was falling asleep later.

A warm night can therefore work in two ways at once:

At night: you fall asleep later or sleep more restlessly.

The next day: you are more tired, less focused and possibly more quickly irritated.

Anyone experiencing several hot nights in a row often feels not just the current daytime heat, but also the missing recovery from previous nights.

You can find concrete measures in our guide Sleeping in the heat: tips for warm summer nights.

7. Why heat sometimes hits the circulation

When blood vessels in the arms, legs and skin widen, blood pressure can drop. At the same time, the body loses fluid through sweating.

In sensitive people this combination can lead to dizziness or weakness. The BIÖG describes, for example with heat collapse, that the widening of blood vessels can lead to a drop in blood pressure and, in pronounced cases, to reduced blood flow to the brain with brief loss of consciousness.

This does not mean that any mild dizziness in the heat is a medical emergency. But it shows why it makes sense to take early signals seriously.

If you feel dizzy:

• stop the physical activity
• go to a cooler place
• sit or lie down
• drink, provided you are conscious and no medical restriction speaks against it
• cool yourself down gently

For severe, unusual or persistent symptoms you should seek medical advice.

8. What really helps the body in the heat?

The most effective measures are surprisingly unspectacular. The BIÖG's official heat guide recommends, among other things, moving physical activities to cooler times of day, drinking enough, avoiding direct sun and refreshing the body with water.

For everyday life this means:

Drink regularly

Not just once you are already very thirsty. Water and unsweetened drinks are a good basis for most people.

Reduce midday heat

Strenuous walks, sport or heavy work can be done in the morning or evening if possible.

Use shade and cooler rooms

Direct sun and heated surfaces increase the heat load even further.

Wear light clothing

Loose, breathable clothing makes it easier to release heat.

Build in small cooling breaks

Cool water on the forearms, face, temples or neck can feel pleasant. The BIÖG explicitly recommends, among other things, cool compresses as well as cool water on the neck and forearms. Extreme cold is not necessary for this.

We have put together further options in our guide Cooling the body pleasantly in the heat .

9. Where ChillNeck comes in usefully

ChillNeck addresses exactly one of these simple cooling options: the neck.

The reusable neck cooling ring contains a phase change material at 26 degrees and thereby offers gentle freshness instead of a brief ice-cold shock. It is worn around the neck and works without electricity, batteries or airflow.

This can be practical in everyday life:

• while commuting
• in the office or home office
• while shopping
• while travelling
• on a walk through town
• in the garden
• at festivals
• while relaxing in the shade

ChillNeck does not cool the whole body and does not protect against heat. It also does not stop sweating. But it can pleasantly refresh a spot where cooling can be worked into everyday life without much effort.

Why the neck, forearms and other easily accessible spots are often used for cooling, we explain in the article Which body parts should you cool in heat?.

And if you want to know why ChillNeck is deliberately not ice cold, you can find our guide How does a neck cooling ring feel? Gentle freshness instead of a brief ice-cold shock.

10. What ChillNeck cannot replace

A pleasant feeling of freshness can make a hot moment easier to bear. That is exactly why this distinction matters.

ChillNeck does not replace:

• drinking enough
• shade
• sun protection
• breaks
• a cool room
• adjusted physical activity
• medical help for symptoms

It is a complementary cooling aid, not a medical device.

If you want to know how the cooling works technically, you will find a simple explanation of the PCM principle on the page How ChillNeck works .

11. When does heat become dangerous?

Fatigue or the need for a break are something different from a severe heat-related illness.

Still, you should know the warning signs.

According to the BIÖG, heat exhaustion can involve, among other things, severe weakness, dizziness, nausea, thirst, low blood pressure and a high heart rate.

A heatstroke, on the other hand, is a life-threatening emergency. Possible signs are a very high body temperature, altered consciousness, confusion, seizures or loss of consciousness. In this case, emergency services must be called immediately on 112 and the affected person must be cooled. Unconscious or severely confused people must not be given anything to drink.

Older people, pregnant women, young children, people with pre-existing conditions and those who work physically hard or for long periods outdoors should be especially attentive.

Conclusion

Heat does considerably more to our body than just bringing sweat to our forehead.

The body tries to release excess heat through stronger blood flow to the skin and sweating. In doing so it loses fluid, the circulation is put under more strain, and we can feel tired or drained.

The brain is not left untouched either. Studies show that high temperatures can impair concentration, reaction speed and cognitive performance. At the same time, warm nights make sleep harder. That can further increase fatigue, concentration problems and irritability the next day.

The best strategy therefore consists of several small measures: drink regularly, reduce direct sun, adjust physical exertion, use shade and cooler rooms, and cool yourself down pleasantly in between.

ChillNeck can be a practical addition here. The neck cooling ring offers gentle freshness at the neck, works without electricity and leaves your hands free. But it does not replace basic heat protection measures.

If you want to try gentle neck cooling for warm days, you can explore ChillNeck in Arctic Mint and Pink Frost and in three sizes.

Frequently asked questions

What happens in the body when it is very hot?

The body tries to release heat through stronger blood flow to the skin and sweating. This can affect blood pressure, fluid balance and circulation. With greater heat strain, fatigue, thirst, headaches, dizziness and concentration problems are among the possible effects.

Why does heat make you tired?

Several factors combine. The circulation has to adapt to the heat, sweating causes fluid loss, and warm nights can make sleep worse. As a result, you can feel exhausted more quickly on hot days.

Why is it harder to concentrate in the heat?

Studies show that high temperatures can impair cognitive performance such as attention, processing speed and working memory. Fluid deficiency and poor sleep can further worsen concentration.

Does heat make you irritable?

High temperatures are linked in studies to worse mood and increased irritability. Besides the heat itself, lack of sleep, physical discomfort and other strains can also play a role. Not everyone reacts the same way.

Why do you sleep worse in the heat?

To fall asleep, the body needs to be able to release heat. In very warm nights this works less well. Large studies show that higher night-time temperatures are linked to shorter sleep and falling asleep later.

Why do I get dizzy in the heat?

Heat widens the blood vessels and can lower blood pressure. Sweating causes additional fluid loss. Both can contribute to weakness or dizziness in sensitive people.

Which body parts can you cool in heat?

Especially practical are easily accessible spots such as the neck, face, temples and forearms. Cool water or a gentle cooling aid can feel pleasant there. Extreme cold is not necessary.

Does ChillNeck help against heat?

ChillNeck does not protect against heat and does not treat heat-related symptoms. But the neck cooling ring can offer gentle freshness at the neck and thereby make warm everyday situations more pleasant.

When should I get medical help in the heat?

Severe or unusual symptoms such as pronounced weakness, repeated vomiting, marked confusion, seizures or loss of consciousness must be taken seriously. If heatstroke is suspected, call emergency services on 112 immediately.