Why Does Sleeping with Your Armpits Open Feel Better?
A Reflection on Sleep Thermoregulation and Underarm Ventilation
1. At Some Point, Sleeveless Shirts Disappeared
When I was a child, I slept in sleeveless shirts.
Back then, it felt natural. On hot nights, my arms were exposed and my armpits were not covered. I did not think of the armpit as a particular body part. It was simply a place that appeared when I raised my arm and disappeared when I lowered it.
Then, at some point, I started sleeping in short-sleeved shirts.
It was not something I chose after thinking it through. As puberty began, short sleeves simply felt more natural than sleeveless shirts in a home shared with family. The sleepwear given to me at home also changed naturally from sleeveless shirts to short sleeves. I just wore what was given.
Sleeveless shirts somehow felt like something from my father’s generation. An old-man kind of shirt. Something you could wear at home, but something that no longer seemed to fit my body. Short sleeves covered the armpits, looked neater, and at some point became the obvious form of sleepwear.
So I slept in short sleeves for a long time. The habit formed while living with family stayed with me even after I began living alone. Even when there was no longer anyone to see or care, the idea of sleeping without a top did not occur to me for a long time. A short-sleeved shirt was simply what I wore to bed.
Then one day, I slept without a top.
The feeling was different. It was not merely that I felt a little less hot. It felt as if some part of my body had opened. My back and chest felt lighter, and the closed sensation around my armpits loosened. The room was the same, the blanket was the same, the night was the same, yet my body felt more comfortable.
That was when a question appeared.
Why does sleeping without a top feel better? Is the difference only the removal of one layer of fabric? Or does the body already have places where heat and moisture need to escape?
Following that question led me, unexpectedly, to the armpit.
2. Why the Armpit?
The armpit is not just the skin under the arm. It is a part of the body where the arm and torso fold into each other and close.
The face is already open. The neck is relatively exposed to air. The hands and feet can be opened immediately by taking them out from under the blanket. The armpit is different. The moment the arm comes down, it folds between the torso and the upper arm. A short sleeve covers it once more, and the blanket covers it again.
That makes the armpit one of the upper body’s easiest ventilation points to close during sleep. It is small, but its conditions are powerful. It sits close to the torso, is pressed by the arm, covered by clothing, and buried under bedding. It is not an open surface of skin. It is a closing gap.
Heat remains more easily in closed spaces than in open ones. Sweat stays longer where air does not flow. Moisture lingers in folds more than on broad surfaces. Friction increases more on damp skin than on dry skin. The armpit is where these four conditions overlap.
Sleeping without a top felt better not simply because one layer of fabric disappeared. One of the body’s most easily closed pockets of heat and moisture had opened. That pocket was the armpit.
That is why this essay looks not at the shirt, but at the armpit. Comfort during sleep depends less on how much of the body is covered and more on how open the closing parts of the body are allowed to remain.
3. Sleep Begins as the Body Lowers Its Inner Heat
Sleep may look like consciousness turning off, but from the body’s perspective it is a shift in thermoregulation.
During the day, the body moves, thinks, works, eats, and responds. Muscles generate heat, organs continue metabolism, and the brain remains alert. At night, the body enters recovery mode. One of the important changes in that transition is a drop in core body temperature.
Research by Kräuchi and colleagues reported that before sleep, blood vessels in distal skin regions such as the hands and feet dilate, increasing heat loss from those regions, and that this process is linked to faster sleep onset. The distal-proximal skin temperature gradient—the temperature difference between distal skin and more central body regions—was presented as a strong predictor of sleep-onset latency.[1]
The meaning is simple.
Before sleep, the body sends internal heat outward. The hands and feet are doors through which that heat can leave. As core body temperature drops, the body enters sleep more easily.
Sleepwear is not just fabric that covers the body at night. Sleepwear is a structure that controls the pathways through which body heat escapes.
Research on the sleep thermal environment points in the same direction. Thermal conditions strongly affect sleep; both heat and cold can increase wakefulness and reduce REM sleep and slow-wave sleep, and these effects change depending on clothing and bedding.[2] A 2024 systematic review also concluded that sleepwear and bedding materials can influence sleep quality through skin temperature, body temperature, and thermal comfort.[3]
Sleep is the time when the body releases heat. Clothing can either open the path for that heat to leave or block it.
4. A Short-Sleeved Shirt Is Clothing That Closes the Armpit
A short-sleeved shirt feels natural as sleepwear. It is easy to put on, covers the body moderately, and looks neat.
Viewed from the structure of the body, another side appears.
A short-sleeved shirt covers the torso, but it also covers the armpit. The sleeve appears to be the part that wraps around the upper arm, but in practice it narrows the entrance to the armpit. When a person lies down with the arms lowered, the sleeve remains around the armpit, reducing airflow between skin and fabric.
A short-sleeved shirt gives neatness. A sleeveless shirt opens the armpit. Sleeping without a top opens both the torso and the armpit.
That difference changes the comfort of sleep.
The reason sleeveless shirts disappeared after puberty can also be seen again from this angle. Sleeveless shirts did not disappear because they were functionally lacking. In a household shared with family, clothing that exposed the armpits naturally moved into the background, and short sleeves became the more natural form of sleepwear.
Culture made the armpit something to cover. The body says it feels better when the armpit is opened.
5. The Five Roles of Underarm Ventilation
Underarm ventilation is not merely about feeling cool. When the armpit is open, heat, sweat, moisture, friction, odor, and skin conditions on the upper body all change during sleep.
First, heat escapes.
The armpit is close to the torso. It closes between the arm and body, then is covered again by clothing and bedding. When this area opens, hidden heat in the upper body gains a path out. Even when the room temperature stays the same, opening the armpit can change how the body feels.
Second, sweat dries.
Coolness is not created by temperature alone. When sweat evaporates, the skin loses heat. If sweat remains on the skin, the body feels sticky and heavy. When air flows, sweat dries, and perceived heat drops in the process.
Third, moisture decreases.
The armpit is a skin fold. Medically, skin folds are areas where moisture and friction easily accumulate. Intertrigo is described as inflammation that occurs between opposing skin surfaces because of moisture, friction, and poor ventilation.[4] StatPearls also explains that warmth, friction, moisture, maceration, and insufficient ventilation can cause or worsen intertrigo in flexural areas such as the armpits.[5]
Fourth, friction decreases.
Dry skin and damp skin do not rub in the same way. When sweat does not dry, clothing stops being a protective layer and becomes a friction layer. During sleep, the body continues to make small movements. When damp fabric repeatedly rubs against skin, irritation grows.
Fifth, the conditions for odor and skin trouble decrease.
Odor and skin problems do not arise from sweat alone. They grow when sweat, sebum, dead skin cells, bacteria, fungi, moisture, friction, and occlusion overlap. The American Academy of Dermatology explains that clothing or gear can trap heat and sweat against the skin, and that rubbing under those conditions can contribute to acne mechanica.[6] A review on truncal acne also identifies clothing friction, sweat retention, and pressure as aggravating factors for acne on the torso.[7]
Underarm ventilation is the work of organizing heat and moisture in the upper body during sleep.
6. Rediscovering the Sleeveless Shirt
A sleeveless shirt can look outdated. It can look like something from a father’s generation. Something worn at home. Something old-fashioned. Something one may not want to return to.
But from the structure of the body, the sleeveless shirt is surprisingly precise.
A short-sleeved shirt covers both the torso and the armpits. Sleeping without a top opens both the torso and the armpits. A sleeveless shirt sits in between.
It covers part of the torso. It absorbs some sweat and sebum from the chest and back and reduces the burden of direct contact between skin and bedding. At the same time, it keeps the armpits open. In that sense, the sleeveless shirt becomes a middle point between torso coverage and underarm ventilation.
A sleeveless shirt is not merely an unfashionable undershirt. It is a structure that keeps the torso partially covered while opening the armpits.
Sleeping without a top is the coolest option. It also provides the greatest underarm ventilation. If bedding can be cleaned regularly, sleeping bare-chested is a strong choice.
A sleeveless shirt is the practical choice. It can work for people who tend to feel cold, and for people who do not want to sleep fully bare-chested. The key is that it does not close the armpits.
The standard for sleepwear is not whether it looks good. The standard is whether it blocks the body during sleep or lets it open.
7. Why the Bottom Stays and the Top Opens
The top and the bottom are not the same kind of clothing. Their roles are different.
Underwear on the lower body is a hygiene barrier. The anus, genitals, bodily discharge, residual urine, and sweat are involved. From the perspective of bedding hygiene, keeping lower-body underwear on is reasonable. Clothing in this area has more to do with hygiene than thermoregulation.
The upper body is different.
The chest, back, and armpits do not carry the same bedding-contamination risk as the genitals. For these areas, heat, moisture, friction, and ventilation matter more. The armpit in particular is a body part that closes by itself, so even a small amount of fabric can leave heat and moisture trapped.
The balance of sleepwear can be summarized simply.
Keep the bottom for hygiene. Open the top for heat and moisture.
The bottom protects the bedding. A bare upper body or a sleeveless shirt opens the armpits.
8. People Who Run Cold and People Who Run Hot
Bodies are not all the same. Some people run hot. Some people get cold easily. The answer to sleepwear changes according to a person’s heat profile.
For people who run hot, sleeping without a top fits well. Heat and moisture trapped around the back, chest, and armpits decrease. Even under the same blanket, the body breathes more easily.
For people who run cold, a sleeveless shirt fits well. The torso can be protected by a blanket and a thin layer of clothing, while the armpits remain open. The important thing here is not to chill the entire upper body. The important thing is not to close the armpits.
Both approaches share the same principle.
Do not close the armpits.
People who run hot open the upper body. People who run cold protect the torso while opening the armpits. The methods differ, but the principle is the same. Sleep becomes more comfortable when the body is not obstructed from doing what it needs to do at night.
9. Conclusion — The Armpit Is the Night’s Vent
The sleeveless shirt that once felt natural in childhood disappeared at some point. As puberty began, the armpit became a body part that felt awkward to show. The short-sleeved shirt became the natural sleepwear that covered it.
That shift made sense socially.
From the body’s perspective, the story is different.
Before the armpit is something to hide, it is something that opens and closes. When it closes, heat gathers, sweat remains, and moisture lingers. When it opens, heat escapes, sweat dries, and the body feels more comfortable.
A short-sleeved shirt gives neatness. A sleeveless shirt opens the armpits. Sleeping without a top opens both the torso and the armpits. Once this difference is felt in the body, sleepwear is no longer just clothing. It becomes a device for nighttime thermoregulation.
What matters during sleep is not covering the upper body neatly. What matters is not interfering with the work the body must do at night.
The armpit is the body’s closed pocket of heat. And sleep becomes more comfortable when that pocket opens.
10. References
- [1] Kräuchi K, Cajochen C, Werth E, Wirz-Justice A. “Functional link between distal vasodilation and sleep-onset latency?” American Journal of Physiology-Regulatory, Integrative and Comparative Physiology. 2000. https://pubmed.ncbi.nlm.nih.gov/10712296/
- [2] Okamoto-Mizuno K, Mizuno K. “Effects of thermal environment on sleep and circadian rhythm.” Journal of Physiological Anthropology. 2012. https://pmc.ncbi.nlm.nih.gov/articles/PMC3427038/
- [3] Li X, et al. “How do sleepwear and bedding fibre types affect sleep quality? A systematic review.” Journal of Sleep Research. 2024. https://pubmed.ncbi.nlm.nih.gov/38627879/ ; https://pmc.ncbi.nlm.nih.gov/articles/PMC11596996/
- [4] Kalra MG, Higgins KE, Kinney BS. “Intertrigo and Secondary Skin Infections.” American Family Physician. 2014. https://www.aafp.org/pubs/afp/issues/2014/0401/p569.html
- [5] Nobles T, Miller RA. “Intertrigo.” StatPearls. Updated 2024. https://www.ncbi.nlm.nih.gov/books/NBK531489/
- [6] American Academy of Dermatology. “Is sports equipment causing your acne?” https://www.aad.org/public/diseases/acne/causes/sports-equipment
- [7] Truncal acne review: “Truncal Acne: An Overview.” Journal of Clinical Medicine. 2022. https://www.mdpi.com/2077-0383/11/13/3660
- [8] DermNet. “Malassezia folliculitis.” https://dermnetnz.org/topics/malassezia-folliculitis
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