Duvet memes and hard science: why do we feel hot, then cold, at night?
There is a famous internet meme about “night-time climate control”: to escape the heat, you stick one leg out from under the duvet. And yet somehow the perfect balance never quite works — we either freeze towards morning or wake up in a sweat.
How the brain and the body argue about temperature
1. Evening and deep sleep: the brain cools down
Before we fall asleep, the body starts actively widening the blood vessels in our hands and feet to release excess heat. At this point the skin may feel warm, but the body’s core temperature is falling and the brain itself is cooling rapidly. This cooling is the main biological signal to fall asleep.
In deep slow-wave sleep the metabolism slows down and the brain’s temperature stays at its lowest, so that the nervous system can fully rest.
2. REM sleep: the climate control switches off
Towards morning, when we have our most vivid dreams (REM sleep), things change sharply:
The brain warms up. Blood flows to it, and its activity rises to waking levels.
The body loses control. Scientists have found that during REM sleep mammals almost completely switch off temperature self-regulation: for a while the body stops responding to cold by shivering and to heat by sweating (Cerri, 2021).
At this point we are defenceless against the temperature of the room. If the bedroom has grown cold towards morning, the body quickly cools down, and we wake up shivering as we come out of the dream phase.
The physics of the meme: why sticking a leg out really works
That meme about the leg is entirely scientific. Research by chronobiologists shows that our palms and soles are the body’s main “radiators” (Kräuchi et al., 2006).
Our hands and feet contain a unique network of small vessels called anastomoses, designed specifically to release heat quickly. When you feel stuffy, a foot outside the duvet instantly cools the blood flowing through it. This cooler blood returns to the internal organs, your overall temperature drops, and the brain gets the signal: “All is well, no overheating, keep sleeping.”
Practical tips for perfect sleep
To keep night-time temperature swings from waking you, three rules are enough:
Keep the bedroom cool: the best air temperature for sleep is 18–20°C. In cool air it is much easier for the body to release internal heat and cool the brain. The best humidity is 30–50%.
Use layers: instead of one thick winter duvet, use a light duvet with a throw on top. If you start to feel chilly towards morning, you can easily pull the throw back up, and if you get hot, you can kick it off with one move.
Choose “breathable” fabrics: bed linen made of natural cotton or linen doesn’t hold moisture, helping the body evaporate sweat as its temperature shifts.
A life hack: how to fall asleep faster by… yawning?
If you have been tossing and turning and can’t fall asleep because it feels stuffy, try a proven method — deliberate yawning. Yawn deeply 10–15 times in a row, opening your mouth wide and breathing in deeply.
How does it work, scientifically?
When you breathe in deeply during a yawn, cool air chills the blood vessels in the nose and mouth (including the deep arteries behind the jaw). The cooled blood then flows straight to the brain, lowering its temperature (Massen et al., 2014). And a cooler brain is the fastest and most powerful trigger for falling asleep.
By the way, if you yawn a lot during the day, it is a sign that your brain has simply overheated from tiredness or hard work and is desperately trying to cool itself down to get your concentration back. At such moments it is best to splash your face with cool water or take a few deep breaths by an open window.
Works mentioned in this article
- Cerri, M. (2021). Thermoregulation and Sleep: Functional Interaction and Central Nervous Control. Comprehensive Physiology. (A study of how the body’s thermoregulation switches off during REM sleep.)
- Kräuchi, K., et al. (2006). Thermoregulatory changes around the time of sleep onset. Physiology & Behavior. (The scientific basis for the role of the hands and feet as the body’s main radiators before sleep.)
- Gallup, A. C., & Gallup, G. G. (2007). Yawning as a Brain Cooling Mechanism. Evolutionary Psychology. / Massen, J. J., et al. (2014). A thermal window for yawning in humans. Physiology & Behavior. (Controlled studies showing that yawning regulates temperature and that a deep yawn cools the brain.)
Psychologist, MSc in Psychology · 7 years in practice
Editor-in-Chief of PSY.one.
