Hot, or wet? The two sleepers who both say they run hot

Host: I run hot at night. Every person I have asked says get a cotton duvet, because cotton is lighter and lighter has to be cooler. Is that right?

Circadian: Sometimes. It depends on what is making you hot, and there are two different things people mean by that word.

Host: They feel the same to me. I kick the covers off either way.

Circadian: That is the trap. One is a room problem and one is a moisture problem. Either your bedroom sits above the temperature you sleep well at, so you are warm all night in a fairly steady way. Or your body is putting out moisture overnight, from hormones or medication or a hard workout, and you wake up flushed and slightly wet. From outside the bed, both of those look like a person kicking the covers off.

Host: How do I tell which one I am?

Circadian: Reach for the sheet when you wake up, before you are properly awake enough to talk yourself into an answer. If it is warm and dry, you have a room problem. If it is damp, or your neck and chest are damp, you have a moisture problem. That one question changes which duvet you should be looking at, and almost nobody asks it.

Host: Say I am the damp one. What does wool do that cotton does not?

Circadian: Wool takes moisture in as vapour, into the middle of the fibre, and lets it back out slowly across the night. It will hold up to about thirty percent of its own weight in moisture and still feel dry when you put your hand on it. So the sweat is being moved and stored somewhere that is not against your skin.

Host: I have to push back on that. Holding a third of its weight in water and feeling dry sounds like something a wool company would say.

Circadian: It is a fair thing to be suspicious of, and the wording is doing real work. The moisture goes inside the fibre, not onto the surface where your skin is. Cotton takes on moisture too, and plenty of it, but it holds that moisture as liquid within the weave, and the weave is the part touching you. Same amount of sweat, two different places for it to end up. That is the whole difference, and everything else people say about these two fibres is downstream of it.

Host: Is there anything behind that beyond fibre lore?

Circadian: There is one study I keep coming back to. Thirty six adults, a sleep lab with proper monitoring, and everyone slept under both fibres on separate nights at thirty degrees Celsius. Same room, same people, same temperature, so the fibre was the only thing that changed. In the older group, people fell asleep in about twelve minutes under wool and about twenty seven under cotton.

Host: Thirty six people is not many.

Circadian: No, it is not, and I would not want anyone buying a duvet on that number alone. It is one night per condition, so it cannot tell you what happens in week three. It measured how long people took to fall asleep, which is a narrower thing than how you feel at four in the morning. And thirty degrees is warmer than most bedrooms, chosen to make the effect visible. What the design does well is strip everything else out, and that is rarer than it sounds in bedding research, where most of what gets quoted is a company testing its own product.

Host: So when is cotton the right call?

Circadian: When the heat is coming from the room rather than from you. A cotton duvet is lighter and the weave is more open, so heat leaves quickly instead of being managed through the night. If you are warm but dry, that is the simpler answer and the better one, and there is no reason to pay for a mechanism you are not using. It also washes at home, which over ten years is not a small thing.

Host: So what does wool cost you, apart from money?

Circadian: Three things worth knowing before you commit. It wants spot cleaning or dry cleaning rather than a wash cycle, so it asks more of you over the years it lasts. It is denser in the hand, and some people read that density as heavy on the first night before they get used to it. And it is an animal fibre, which rules it out for some households on grounds that have nothing to do with sleep.

Host: What about people who are both? Warm room and night sweats.

Circadian: The moisture is the one that wakes you up, so solve that first, then bring the room down with a fan or a thinner cover over the top. Airflow rarely fixes damp. Handling the damp often takes enough of the heat with it that the room stops being the problem you thought it was.

Host: Last one. If someone remembers a single sentence from this, what should it be?

Circadian: Work out whether you are hot or whether you are wet, because the two want opposite bedding, and most people buy for the wrong one. If you take the sheet test for three mornings and it comes back dry every time, stop reading about wool and go and fix your room. If it comes back damp, the fibre is the lever you have.

Host: Ours are the Organic Wool Duvet and the Organic Cotton Duvet, and both are certified to the organic textile standard as finished duvets, the fill as well as the shell.

Circadian: The written version of everything we said is at circadianrest.com/blogs/pillow-talk/wool-vs-cotton-duvet-hot-sleepers, along with the study. Our voices in this episode are synthetic. The words are ours.