Why does being outside make you tired

The thermometer has crept far into the minus numbers, and you head out into the winter weather. You take it easy. Maybe you take a short walk or stand still and talk to someone you met along the way.

So how does the cold affect your body? Does cold weather actually make you more exhausted — just because it's cold?

“The simple answer is no, as long as you’re good and warm. If you wear good clothes and your body doesn’t have to work extra to keep warm, then you don’t expend any more energy in cold surroundings,” says Øystein Wiggen, a researcher at SINTEF, an independent research institute.

But it can quickly change if you’re not dressed well enough, Wiggen says.

When you feel like you’re freezing, it’s a sign that your body is losing more heat than it can produce.

Then you begin to shiver, which has a very special function.

Øystein Wiggen studies how the human body reacts to cold and what kinds of products can be used to protect us in extreme environments. (Photo: SINTEF)

“Shivering is the body's defence against cooling,” says Erik Sveberg Dietrichs, a medical doctor at Diakonhjemmet Hospital and an associate professor at UiT — The Arctic University of Norway.

He studies whether heart medicines work just as well for people who have been severely chilled after falling into icy water or being taken by an avalanche.

“The muscles shiver to produce heat. Then they burn nutrients. But instead of allowing you to move, they only produce heat,” says the researcher.

When you stand still and shiver, your muscles are transformed into pure heaters. This is unlike when you move, where some of the energy goes to movement, and the rest is heat.

But do you actually get exhausted from shivering because you’re freezing?

Research subjects shivered for hours

Volunteer participants have sat still for hours in a cold laboratory at SINTEF, while researchers have measured their skin and core temperatures and oxygen uptake.

This allows researchers to follow how low the internal and external temperatures of the participants’ bodies get. And they can also see how much heat the volunteers produce when they shiver.

“People get exhausted from shivering, even if they have just sat still,” Wiggen says about the experiments.

Nerve signals to the brain

And it's not just shivering that makes us more exhausted if we freeze.

Dietrichs says that our body has several defence mechanisms against cold. Our core temperature should preferably remain around 37 degrees C.

“If your skin gets cold, the nerves send signals to the temperature system in the brain,” he said. “The brain does everything it can to prevent us from getting too cold.”

Blood vessels in the muscles contract

In addition to the fact that the muscles begin to shiver, the blood vessels inside them contract, Dietrichs says.

At this stage, the vital organs, such as the brain, heart and lungs, must be prioritized.

If your muscles get less blood supply, they work a little worse. That may mean you may not be able to do perform as well as you usually do.

Another defence the brain uses to fight the cold is to direct the body to deal with the situation, he said.

“You will instinctively seek warmth and try to move when you start to freeze,” he points out.

Lots of heat from movement

Physical activity is an effective way to stay warm — which is something that everyone who has tried to keep from freezing in the cold knows.

But shivering seems almost illogical. Because if shivering turns your muscles into pure heaters, shouldn't you just stand still and let them do their thing?

Øystein Wiggen explains why this is not a good idea.

Your muscles can create much more heat when you are physically active than when you sit and shiver, even if some of your energy also goes to movement.

Erik Sveberg Dietrichs is a medical doctor and researcher. He has written a book about what happens inside the body when humans are exposed to extreme conditions. (Photo: Jan Fredrik Frantzen)

Dietrichs uses himself as an example. He jogs to work in the winter, without getting cold or feeling more exhausted than he usually does when it is hot outside.

Because it can get too hot — in the summer.

There are deaths during marathons that are due to overheating, the researcher points out.

From pain to numb

Both researchers also add that it’s important to be aware that we can get frostbite when it is very cold.

“It can get so cold that your protective system is destroyed. If you get frostbite, your body’s sensors may stop working, and your body will not begin counteractive measures,” Dietrichs said.

The sensors that detect cold are mainly found in the skin. And this is also where frost can do its damage.

“First you feel cold, then you start to hurt and then you turn numb. Then the risk of frostbite increases because you can’t feel your extremities in the same way,” says Wiggen.

Two more things can make you more tired in the cold.

One is from wearing a lot more clothes, which means that you have to use a little more energy to move.

The other is if you have cold-induced asthma, you will be more tired.

Translated by: Nancy Bazilchuk

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Read the Norwegian version of this article on forskning.no

Temperature records are being shattered all over the United States this summer. Yet as this heat intensity rises, beaches and parks aren’t the only areas filling up with people — emergency rooms are, too.

Ever notice how the sun makes you tired after a day spent outdoors? The Mayo Clinic tells us this sleepiness is an early sign of heat exhaustion, a condition that can escalate to life-threatening heat stroke. A 2021 study published in The BMJ links today’s increasingly hot summer days with more hospital visits for adults of all ages. 

Now, sun exposure has plenty of benefits. It keeps your sleep cycle sound, regulates your mood and may even boost your lifespan. But research points to some important reasons why the sun makes you sleepy, too, and why you shouldn’t ignore them. 

Dehydration from the Sun Makes You Tired

Whether you’re sitting poolside or hiking a mountain trail, the sun dehydrates your body, according to the National Sleep Foundation. And that overwhelming fatigue after a summer day outdoors is one of the tell-tale signs your body’s fluid balance is out of whack.

The danger this poses goes beyond feeling extra thirsty. A 2019 review published in Nutrients warns that losing just two percent of your body water messes with your skin health, mood, memory, concentration, kidneys, gut and even weight management.

The authors point out this dehydration risk doesn’t only apply to people performing intense activity in the heat, either. Simply catching some rays on your favorite beach towel causes your body to sweat, draining fluids and salt that keeps your risk for heat exhaustion or heat stroke at bay.

Be mindful of your fluid intake throughout the day and be aware of dehydration symptoms, such as stinky breath and irritability. You can also keep nutritionist-approved electrolyte drinks on hand to replace the salts you lose from sweating.

All that extra time in the heat causes your body to work hard, no matter what you’re doing. The National Sleep Foundation says our bodies work hard throughout the day to maintain a constant internal temperature. So when we expose ourselves to long periods of heat, our bodies go into overdrive to maintain that core temperature. 

That Suntan Makes You Tired

Being in the sun causes chemical changes in our bodies. Just look at your suntan (or sunburn) for proof. And those UV rays cause some serious damage. When we spend too much time in the sun, the same chemicals that tan our skin can also cause fatigue. 

To fight back, The National Sleep Foundation recommends staying inside during the hottest hours of the day — typically between noon and 3 p.m.— and taking frequent breaks in the shade at other times. Balance your activity balanced with plenty of rest, and don’t forget to pack along a sun hat or umbrella for additional cover.

If you limit your sun exposure and remain mindful of possible sunburn, you should be able to spend a day outdoors without feeling like you need a nap afterward.