The short answer
Sleep comes fastest when the room is cool — most sleep researchers point to somewhere around 18°C — and the bed is warm. Not the air. The bed. A cool room lets your core temperature fall, which is the signal your body waits for before it lets go. A warm bed opens the blood vessels in your skin, hands and feet, which is how the core sheds that heat in the first place. You need both. Most bedrooms in Europe get one of them.
Why the room should be cool
Your core temperature runs on a daily curve. It peaks in the late afternoon and drops by about a degree through the evening, bottoming out in the small hours. Sleep onset rides that downward slope. A bedroom heated to 22 or 23°C works against it: the body cannot dump heat into air that is nearly as warm as its own skin, so the curve flattens and you lie there, tired and wide awake.
This is why "sleep in a cold room" became standard advice. It is good advice. It is also where most people stop.
Why the bed should be warm
Here is the part that sounds backwards. To cool the core, the body has to warm the surface. Blood vessels in the hands and feet widen, blood flows outward, and heat leaves through the skin. Warm the skin gently and you speed that process up.
A Swiss research group showed this in 1999 with a finding that still gets quoted: of everything they measured, the temperature of the feet was the best predictor of how quickly people fell asleep. Warm feet, fast sleep. Twenty years later, a large review of bathing studies found the same pattern from a different angle — a warm bath or shower one to two hours before bed shortens the time it takes to fall asleep. The warmth is not the point. The letting-go that follows it is.
A cold bed does the opposite. Your body clamps the vessels shut to protect the core, and now it is holding heat in exactly when it needs to release it. You know this feeling: cold sheets, curled up tight, waiting for the bed to warm up before you can even think about sleep.
What Korea worked out first
Traditional Korean houses were built around a system called ondol (온돌), which dates back roughly three thousand years. The kitchen fire sends heat through stone channels — the gorae — under the floor of the sleeping room. The floor becomes warm. The air above it stays cool, because Korean winters are cold and the houses were never sealed.
People slept on that floor, on thin bedding, with cool air on their faces and warmth along their backs. Nobody wrote a paper about it. They just noticed that it worked, and kept building houses that way for a hundred generations. Modern Korean apartments still heat the floor rather than the air, and the phrase for pressing your back against a hot floor — 등을 지지다 — is still something you say when you are tired and want to be looked after.
Cool air above, warmth below. The research caught up with the floor.
How to do it in a European bedroom
You do not need a stone floor. You need to separate two things most people treat as one: the temperature of the room and the temperature of the bed.
Turn the room down. Radiator off or low, window cracked if the street allows it. Aim for the point where the air on your face feels fresh, not the point where you shiver undressing. For most people that is 17 to 19°C.
Warm the bed, not the air. There are three ways to do this, and they are not equal. A hot-water bottle warms one patch of surface and is cold by two in the morning. An electric blanket warms the whole surface, but by heating a wire against your skin — it is the skin that gets hot, which is why so many of them get kicked off in the night. A far-infrared topper warms the body rather than the surface: the fabric stays comfortable while the warmth goes in, so the room can stay cool and the bed can stay warm all night.
Lighten the duvet. If the bed is warm from underneath, you need less on top. A heavy duvet in a cool room traps the heat you are trying to release.
Give it two weeks. Your body has a set point for what "warm enough" feels like, and it moves. Start gentle — with Litual we suggest 27°C on the first night — and lower it a degree at a time once you are in bed. Most people end up lower than they expected.
What it feels like when it works
Cool air at the tip of your nose. Warmth along your back and under your legs. Your shoulders come down a little. You stop noticing the bed at all, which is the whole idea. The Korean word for this is not "hot" and not "cozy". It is 온기 — ongi — the warmth that is simply there, holding you, asking nothing.
Questions people ask
Is 18°C too cold for a bedroom?
Not if the bed is warm. The cool air is what lets your core temperature drop; the warm bed is what stops your body from fighting it. If you shiver in bed at 18°C, the problem is the bed, not the room.
Won't a heated bed make me overheat?
Surface heat can — an electric blanket warms the skin directly, and overheated skin wakes you. Radiant far-infrared warmth goes into the body while the surface stays comfortable, which is why it can stay on with a timer without the 2am kick-off.
What about summer?
The principle holds. Keep the room as cool as you can, and use a lighter, breathable surface. Nine in ten Litual owners keep using it through summer, usually at a lower setting on the airy side of the topper — not to get warm, but because the body still relaxes into it.
Is this the same as sleeping with socks on?
It is the same mechanism, smaller. Warm socks warm the feet, the feet widen their vessels, the core cools. A warm bed does it for the whole body.
This piece is about comfort and sleep habits, not medical advice. Litual is a bedding product, not a medical device. If you have a condition that affects how you sense or regulate heat, talk to your doctor before changing how you warm your bed.