Cold

When sleep goes badly, the usual suspects are all reasonable ones. A mattress past its life. A meal eaten too late. A day that will not stop replaying itself. Coffee at four in the afternoon, a phone in bed, a child, a noise, a worry. Each of those is worth fixing on its own terms, and any of them can ruin a night.

Temperature tends to come up later, if it comes up at all, and there is a fair reason for that. A bedroom that feels comfortable when you get into it seems, reasonably enough, like a bedroom that is set correctly. But those are two different questions. A room that feels pleasant to someone standing up in it, awake and dressed, is usually a few degrees warmer than the same room needs to be for the thing that has to happen next.

And unlike a mattress or a late supper, it can be settled tonight, for nothing. Turn the heating down, add a blanket, and see what the night does.

What has to happen for sleep to begin

Sleep does not start in the head. It starts as a change in temperature. In the hour or so before you drop off, your body begins deliberately shedding heat: the blood vessels in the skin of your hands and feet open up, warm blood arrives at the surface, and heat radiates away into the room. Your core temperature falls by something close to a whole degree, and it is that fall, not the hour on the clock, that opens the door to sleep.

Which explains the leg. If the room is too warm to accept the heat you are trying to give it, the body improvises. It puts a foot out into the coldest air it can reach and radiates from there instead, because a bare foot in cool air is a remarkably efficient radiator. The duvet gets kicked back at three in the morning for the same reason. None of that is restlessness. It is a body doing the arithmetic correctly in a room that is working against it.

The reverse case is just as consistent. Reviews of the research on thermal environment and sleep find that heat, especially heat with humidity in it, increases the time spent awake and cuts into both slow-wave sleep and REM. The night does not simply get shorter; it gets shallower, which is why a hot night can leave you flattened after what looked like eight hours in bed.

Why the obvious version of this is wrong

Told that a cool room helps, most people reach for the wrong version of cool. They make themselves cold, and they are then surprised when it does not work.

Cold feet do not help you sleep. Warm ones do. In a study published in Nature, warming people's feet and hands sped up the onset of sleep, because warming the skin is what opens those surface vessels and lets the core heat escape. Cold extremities do the opposite: the body clamps the vessels shut to conserve heat, the core cannot unload, and you lie there awake with cold feet and a warm middle. So socks are not a contradiction, and neither is a hot water bottle at the feet. The bed can be as warm as you like. It is the air of the room that has to be cold.

The number worth aiming at is around 18 degrees, with roughly 16 to 19 the sensible range for an adult under normal bedding. It will feel too cool when you walk in, and that is the point: you are not setting the room for a person standing in it, you are setting it for a person lying still under wool.

The second is to count on the open window in summer. A draught does help, but on a hot night the air outside is warm too, and opening the window wider will not change that. Take a blanket off the bed instead.

And plants will not do it for you. A bedroom full of them looks like fresh air, but after dark plants breathe in the same direction you do, taking in oxygen and giving off carbon dioxide. A few pots make little difference either way. It is simply one more reason to sleep with the window open rather than leave the job to the greenery.

What the best rooms already do

Anyone who has slept well in an expensive hotel has felt this without naming it. The rooms at the top of the market run cool, quietly, all night, with heavy bedding to compensate and no fan noise anywhere. It is one of the few luxuries in a suite that costs the operator almost nothing to provide and that guests reliably describe afterwards as the best sleep they have had in months.

At home you are buying nothing at all. You are turning a dial the wrong way on purpose and putting another blanket on the bed.

How to cool a bedroom, in order

Cheapest and most effective first.

• Turn the bedroom heating down to about 18 degrees, and leave it there for a week. Free, immediate, and the single change that does most of the work. Do not judge it on one night; give the body a few nights to stop expecting the old room.
• Put the warmth on the bed instead of in the air. One more blanket of wool or cotton, and socks or a hot water bottle if your feet are cold. Warm body, cold room. This is the half of the instruction that people skip, and it is the half that makes the cold room pleasant rather than punishing.
• Air the room before you get into it, and again when you get up. Five minutes of an open window empties out the night's breathing and drops the temperature at the same time. It costs nothing, even in winter, because the walls and furniture stay warm. There is more on this in the piece on air in a closed room.
• Close the radiator, not the door. A bedroom radiator turned down or off, with the door left open to the rest of the house, gives you a cool room that never gets stuffy. A closed door with the heating on gives you the worst of both.
• Deal with the sun before it arrives. In summer, shutters, blinds or curtains closed during the day keep several degrees out of a bedroom that faces the sun, and they cost nothing to close. This is where cool and dark start doing each other favours: see how dark a bedroom actually needs to be.
• Lose the synthetic layers. Polyester bedding and foam mattresses hold heat and hold damp, which is exactly the combination the body is trying to escape. Wool, cotton and linen move moisture away and let the temperature settle.
• Only then consider buying anything, and let it be bedding. Layers you can add and take away are what actually regulate a night: the mattresses and bed linen I would sleep in are in the bedding group of the Hearth directory.

The test is your own foot. If it is out from under the duvet by midnight, the room is too warm, whatever the thermostat says.

What changes

Nobody feels transformed by a cooler bedroom. What people report is that falling asleep stops being an activity. You get in, you are warm, the room takes the heat away, and the next thing is morning. The three-in-the-morning wake-up, the one with no obvious cause, is often the first thing to go.

Temperature is part of the standard I hold the bedroom to, and the rest of it is in the third principle of the Hearth: the room that works the night shift gets the strictest rules in the house. Together with darkness and air, it is most of what a bedroom actually owes you.

So turn the dial down, pull up one more blanket of wool or cotton, and let the room go cold around a warm bed. A cool room and a warm body: every creature that sleeps well sleeps like this.