We’ve all experienced it at some point. You’re curled under a duvet, snug as a bug and you should feel fine. Except your feet are ice blocks, and your back is sweating like it’s mid-July. It turns out, this nightly annoyance isn’t just your imagination. It’s science. And it’s weirdly complex.
A group of scientists in Denmark and Japan decided to peek under the covers to figure out how bedding actually keeps us warm (or doesn’t). What they discovered is that sleep comfort is not just about the thickness of your blanket. It’s also about what parts of you are under it and even the position you’re in.
“Even when the same bedding system provides the same [overall] insulation, the local skin temperature changes depending on which body parts are covered or in contact,” the researchers report.
Not All Blankets Are Equal
The researchers used a thermal manikin (sort of like a robot crash-test dummy, but more cozy). They simulated human heat loss across 84 different sleep setups. They changed things like sleeping posture (on your back or side), whether the dummy was nude or pajama-clad, and how much of it was tucked in. Then they added either a cotton blanket or a fluffy duck-feather duvet. All of this took place inside a perfectly climate-controlled chamber.
To quantify how well bedding keeps a person warm, scientists use a unit called “clo” (short for “clothing”). One clo equals the insulation needed to maintain comfort at room temperature while seated. It’s roughly equivalent to wearing a business suit.
The team found that insulation doesn’t spread evenly across your body, even if the bedding technically has the same “overall” warmth. That means your shoulders could be shivering while your hips are at a tropical beach party. And this isn’t about how you perceive things, it’s about how heat is actually distributed through your body.
To make sense of it all, the researchers also worked with the concept of “equivalent temperature.” It’s a bit like saying, “Sure, the room is 22°C, but it feels like 26°C because you’re tucked in like a burrito.” They tested 84 different sleeping conditions — varying posture (back or side), clothing (nude or pajamas), quilt type (blanket or duvet), and how much of the body was covered — across three ambient temperatures (18.6°C, 22.6°C, and 26.4°C).
By comparing how these factors shifted the perceived temperature, the researchers showed that bedding isn’t just a passive layer. It actively shapes how warm or cool your body feels, regardless of what the thermostat says.
What They Found Out
It’s not just how much blanket you have — it’s where you put it that counts. Covering the torso, for example, can help warm up exposed limbs by boosting the body’s core temperature. Meanwhile, leaving areas like the arms or feet uncovered can lead to cold spots that disturb sleep. Even when total insulation remained constant, shifting which parts of the body were covered caused noticeable changes in local skin temperature. This underscores the importance of strategically arranging bedding. Things like tucking in your feet or wrapping your arms can help maintain comfort without relying solely on heating the entire room.
Yes, the study suggests that “the tuck” might be a good idea.
But it’s not just about tucking in your feet. The researchers found that different parts of the body respond differently to insulation, and warming one region — like your core — can sometimes help heat up more distant areas, like your toes. Meanwhile, leaving even small parts uncovered, like your shoulders or arms, can create cold spots that throw off your whole-body comfort.
They also discovered the limits of coziness: too much insulation in a warm room can trigger sweating, even if you feel fine when you fall asleep. The researchers used a thermal simulation model to figure out the exact point at which your body might begin to overheat under different bedding setups. At room temperatures above 22°C, they found that even a medium-weight duvet could push your body into sweaty territory if you’re fully covered.
What It Means for Our Sleep
In short, your bed is a microclimate — and you’re the weather system. The amount of insulation you need isn’t fixed, and it doesn’t depend just on how thick your blanket is. It depends on how you’re sleeping, what you’re wearing, what’s covered, and how warm your room is. A thick duvet might be perfect on a cold night if you’re in pajamas and lying still. But on a warm evening, going bare-legged with just a light blanket over your chest might be the smarter move.
So, the next time you’re tossing off the covers or curling into a blanket burrito, remember: you’re not being picky, you’re thermoregulating. This study doesn’t just explain why your back sweats while your feet freeze — it gives us permission to rearrange our bedding mid-night like experimental physicists of comfort. Sleep, it turns out, is a science of its own — and you’re the lab.
The study was published in the journal Building and Environment.
