Cold
Your foot slipping out from under the duvet is a tell that your body wants to cool down. Why warm feet help you fall asleep, and why the room should stay near 18 degrees during the night.
On a warm night, one leg nearly always finds its way out from under the duvet. A bare foot loses heat quickly, so it slips out when the room or bedding is too warm.
Your body starts cooling before sleep. During the hour before you fall asleep, warm blood moves from the core towards the hands and feet. Once it reaches the skin, heat leaves the body and your core temperature begins to fall.
The best predictor was warm feet
A 1999 paper in Nature measured the physical changes that happen as people fall asleep. The best predictor of rapid sleep was how widely the blood vessels in their hands and feet had opened. Warm feet meant that heat was leaving the body.
This is why cold feet can keep you awake in a cool room. Socks or a hot-water bottle warm the skin and bring blood closer to the surface. The cooler air then carries heat away from the skin.
Warm feet help the body cool.
Cold enough to need the duvet
A review of sleep and temperature studies found that heat increased wakefulness and reduced deep sleep and REM sleep when people used ordinary bedding. Colder air did not disturb sleep stages in the same way. The duvet kept the skin comfortable while the core cooled.
Start at 18 degrees. Most people settle somewhere between 16 and 19. It may feel colder than sensible when you first walk in, but the room is calibrated for the time in bed.
18 degrees is only half the setting
You can optimize your sleep temperature setup even further. A winter duvet and a linen sheet create very different temperatures against the skin, even when the room stays at 18 degrees.
Set the room and the bed separately. Cool air carries heat away while the bedding keeps your skin comfortable.
- Start at 18 degrees. Try it for one week. If you wake hot, lower the room or use a lighter layer. If only your feet are cold, warm them instead of heating the whole room.
- Warm the feet. Socks or a hot-water bottle help cold feet without heating the whole room. Wool, cotton and linen move heat and moisture away from your skin. The mattresses and bed linen I would choose are in the bedding group of the Hearth directory.
- Keep the sun outside. In summer, close shutters, blinds or curtains before direct sun reaches the bedroom. Open the window once the air outside is cooler. If the night stays hot, take a layer off the bed.
- Keep the plants and crack the window. Plants use a small amount of oxygen at night too. Feng Shui also considers their energy too lively for deep sleep. If you love how they look, keep them and crack a window for fresh air. There is more on this in the article about bedroom air.
When the air outside is cooler than the bedroom, open the window.
With our compliments.
| Set it to | Because | |
|---|---|---|
| Air | About 18°C, usually within 16 to 19°C | Your body needs to release heat as sleep begins |
| Bed | Warm enough to feel comfortable | Adjustable layers prevent both cold and overheating |
| Feet | Socks or a hot-water bottle when needed | Warm hands and feet can help the body release heat |
| Heating | Lower than the rest of the home | A bedroom rarely needs living-room warmth |
| Window | Open when the outside air helps | Ventilation replaces stale air and may cool the room |
| Bedding | Wool, cotton or linen | Breathable fibres help heat and moisture move away |
The short answers
The same thing again, in the order people ask it.
What temperature should a bedroom be for sleep?
Around 18°C is a useful starting point, with 16 to 19°C the usual range. It may feel cooler than sensible while you are standing there in clothes. Remember that you are setting it for someone lying under a duvet.
Why does a cool room help you fall asleep?
Your body begins releasing heat before sleep. Blood vessels in the hands and feet open, warm blood reaches the skin and core temperature starts to fall. A cool room lets that heat leave.
Why do warm feet help when the room is cool?
Your body releases heat through warm skin. This is why socks and a cool room belong together. A Nature study found that warming people's hands and feet helped them fall asleep faster.
Can the bed be too warm?
Yes. The bed should feel comfortably warm. If the duvet ends up on the floor or one leg spends the night outside it, remove a layer.
Should you wear socks to bed?
If your feet are cold, yes. Choose a loose, comfortable pair. Warm feet can help you fall asleep while the rest of the room stays cool.
Why do people put one leg outside the duvet?
A bare foot loses heat quickly. The leg outside the duvet is your body going to look for the cold itself.
Does a cool room change sleep quality?
A review of the research found that heat increases wakefulness and can reduce deep sleep and REM sleep. A cooler room helps the body regulate its temperature through the night.
How can you cool a bedroom without air conditioning?
Lower the heating. In summer, close the shutters or blinds before direct sun reaches the room. Air it when the outside temperature helps, then adjust the bedding separately.
Does opening the window help on a hot night?
Yes, when the outside air is cooler or the opening creates a breeze. If the night is as hot as the room, opening the window wider cannot create cold air. Remove a layer from the bed instead.
Do plants use oxygen at night?
Yes, but the amount is small. Even 5 large plants use far less oxygen overnight than 2 sleeping adults. Keep them for their beauty and crack the window for the people in the room.
Does bedding material matter?
Yes. Wool, cotton and linen move heat and moisture away from the body. Use layers you can add or remove without changing the temperature of the whole room.
How quickly should you notice a difference?
Give it one week. One night can be unusual for any number of reasons. Notice how quickly you fall asleep, whether you wake hot, and where the duvet is by morning.
The studies. Okamoto-Mizuno K, Mizuno K. "Effects of thermal environment on sleep and circadian rhythm." Journal of Physiological Anthropology, 2012;31:14. A review of the evidence that heat exposure increases wakefulness and reduces slow-wave and REM sleep, with humidity worsening the effect. doi:10.1186/1880-6805-31-14 · PMID 22738673.
Kräuchi K, Cajochen C, Werth E, Wirz-Justice A. "Warm feet promote the rapid onset of sleep." Nature, 1999;401:36-37. Warming the skin of the feet and hands increased heat loss from the core and shortened the time taken to fall asleep. doi:10.1038/43366 · PMID 10485703.
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