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How Sweat Works: Why We Sweat When We’re Hot, as Well as When We’re Not

Aug. 19, 2020 – Katie McCallum

While most other mammals can sprint faster than we can (a perk of having four legs), humans have basically evolved to be endurance runners. The theory is that this made us more efficient hunter-gatherers.

So what does this have to do with sweat? Well, if it weren’t for how efficiently we produce and dissipate sweat, our ancestors wouldn’t have been able to run long distances while hunting — and we probably wouldn’t be able to do things like run marathons.

But, as you already know, it’s not just a long run that can make you work up a sweat. It’s the little things, too, like being outside on a hot day or taking the stairs instead of the elevator. But, somewhat confusingly, you may also find yourself sweating at night, even though you’re totally at rest, as well as when you’re just scared, nervous or in pain.

Here’s everything you ever wanted to know (or maybe didn’t know you wanted to know) about how sweat works, including why we sweat when we’re hot, as well as why we sweat even when we’re not.

How sweat cools you down

Sweating gets a bad rap, and, sure, it is sort of gross. But sweating is vitally important for body temperature regulation and your overall health.

A person’s internal body temperature generally hangs around 98 degrees Fahrenheit. There’s flexibility here, but if your body gets too hot, whether it’s due to the temperature outside, being physically active or having a fever, bad things can happen — for instance, heat stroke. Fortunately, your body has very sophisticated mechanisms for sensing and regulating body temperature.

As soon as your body’s internal temperature starts rising, your hypothalamus (a small region in your brain) tells eccrine sweat glands distributed all over your body that it’s time to start cooling you down by producing sweat.

Cooling down, however, isn’t as easy as this sweat just dripping off of you. Some of this sweat has to evaporate off of your skin for this process to actually work. That’s because cooling your body via sweating relies on a principle of physics called “heat of vaporization.”

It takes energy to evaporate sweat off of your skin, and that energy is heat. As your excess body heat is used to convert beads of sweat into vapor, you start to cool down.

The other trade off here, though, is that you also lose water as you sweat — and water is critically important for just about every organ in your body. This means that when you’re sweating, you also need to make sure you’re drinking plenty of water so you can replace the water you lose with water you can use.

All this to say, releasing heat through beads of sweat that can easily evaporate off the skin is a very effective way of cooling your body down. By contrast, your dog releases heat by panting — which isn’t nearly as effective as sweating.

But, none of this explains why we sweat even when we’re not hot.

Why we sweat when we’re nervous

Whether it’s your first day of work, a first date or a stressfully close sporting event, I’m sure we’ve all experienced the sweaty palms and underarms that come along with being stressed, scared or nervous.

What you may not realize, however, is that this “emotional sweating” isn’t quite the same as cooling sweat. It happens for a different reason, and it’s primarily associated with a different type of sweat gland altogether.

In response to something stressful, scary, nerve-wracking or anxiety-inducing — and regardless of your body temperature — your body enters what’s called the “fight-or- flight” response. Among other things, this response revs up a second type of sweat gland found on your body: apocrine sweat glands. To be fair, your eccrine sweat glands get activated to some extent too, hence the sweaty palms, but what you’re probably most concerned with are your sweaty armpits — and you have your apocrine glands to thank for that.

Unlike eccrine sweat glands, which are found all over your body and produce sweat that’s mostly made up of water and salts, apocrine sweat glands are isolated to your armpits and groin and produce a thicker, fattier sweat. Why does this matter? Because it’s what gives your armpits that distinctive odor you might smell if you forget to put on deodorant. To be clear, the sweat released from your apocrine glands doesn’t smell itself. But the bacteria that live in your armpits love this oily sweat, quickly metabolizing its nutrients into some pretty smelly byproducts.

So why do we sweat when we’re stressed? It’s probably not a totally satisfying answer, but it’s because we’ve been doing it since Day 1.

Our fight-or-flight response is a hard-wired, inherited response that our ancestors relied on to deal with the various threats to their survival. This response is characterized by a release of adrenaline, as well as other stress hormones, which in turn raises your heart and respiratory rates, increases blood flow and tenses your muscles. It gets your body ready to act, but this adrenaline rush also activates your apocrine glands — and with that comes sweat.

Why we sweat while eating spicy food

We’ve probably all been there. The menu says “VERY SPICY” in a bold font that’s accompanied by five hot pepper symbols, but your order it anyway. You just feel like something spicy!

A few bites in, you can’t feel your mouth anymore.

While you know your mouth isn’t actually on fire, what you may not know is that your brain is actaully getting tricked into thinking you’re overheating. It’s why, pretty soon, you’ll probably be sweating bullets.

As it turns out, the culprit of this so called “gustatory sweating” is capsaicin, the active ingredient that makes many spicy foods spicy.

Capsaicin interacts with temperature-sensitive nerves responsible for detecting warmth in your mouth. This interaction tricks your body into thinking your mouth is literally hot, even though it’s not. Regardless, your body tries to cool you down the best way it knows how — sweating!

But since this sweat may not feel like it’s actually helping tame the fire drill in your mouth, you may be temped to turn to a beverage or other food item to help cool your mouth down instead.

It’s also important to mention here that the proces of metabolizing food, in general, can increase your body temperature. So, even if you’re not eating something spicy and even if you’re not hot, you may find that eating a particularly heavy meal might induce a light sweat — hence the term “meat sweats.”

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