Bamboo viscose, cotton, or polyester: what to sleep in if you run hot

Most pajama shopping is done on the strength of a photograph and a word like "silky". Neither tells you the thing that matters if you run hot at night, which is what the fabric does once you are warm and slightly damp and four hours from morning.

Here is the comparison, without the marketing.

The two jobs

Sleepwear for a hot sleeper has to do two things, and they are different.

Let air through. Air moving across skin is how heat leaves. A fabric that seals is a fabric that traps.

Move moisture away. Not absorb it and sit in it. Move it to the outside of the fabric where it can evaporate, so that your skin dries instead of staying wet.

Almost every fabric is good at one of these, mediocre at the other, or bad at both. Grade them on both jobs.

Polyester and fleece

Breathability: poor. Moisture: does not absorb, does not move.

Polyester is hydrophobic. It does not soak up water, which sounds useful and is not, because untreated polyester also will not carry moisture away. Sweat stays liquid, against your skin, going nowhere. Fleece adds a lofted structure that is specifically engineered to trap a layer of warm air, which is exactly what you want on a chairlift and exactly what you do not want at 3 a.m.

This is most of what is sold as cosy sleepwear. If a set is cheap, soft, fuzzy, and in a holiday print, assume polyester and check the label.

Worth knowing: performance polyester with a genuine wicking finish is a different and much better product. It is also not what pajamas are made of.

Cotton

Breathability: good. Moisture: absorbs a lot, releases slowly.

Cotton is a real improvement and it is why cotton pajamas feel better than fleece ones. Air passes through. Nothing is sealed.

The problem is the second job. Cotton fibres hold a large amount of water relative to their weight, and once saturated they stay saturated for a long time. The practical version: fine at 11 p.m., soaked by 3 a.m., and still clammy when your alarm goes. Then the wet fabric cools in a 66-degree room and you are cold, which is why so many people describe the night as alternating between too hot and too cold rather than simply too hot.

Heavier cotton, flannel especially, is worse on both counts. Flannel is napped to trap air.

Viscose made from bamboo

Breathability: good. Moisture: moves it off the skin and dries quickly.

Bamboo viscose knits into a light, fluid jersey that drapes away from the body rather than clinging. It passes air, and it transports moisture to the fabric surface where it can evaporate, so skin dries rather than staying wet in a damp layer.

It is also genuinely cooler to the touch, which does not affect the physics much but does affect whether getting into bed feels like relief.

The honest caveats, because this is a fabric people oversell:

• It is not magic and it is not a medical product. It gets out of the way of a process your body is already doing.
• It is a manufactured fibre. Bamboo pulp is chemically processed into viscose. The finished fabric is not a plant you are wearing, and brands implying otherwise are stretching.
• It wrinkles, and it does not love a hot dryer.

How to read a label without being had

"Bamboo" on its own is a red flag. The FTC requires this fabric be described as "viscose made from bamboo" or "rayon made from bamboo", and has fined national retailers for dropping the qualifier. A brand that writes plain "bamboo" is either careless or counting on you not knowing. We write it out every time, including when it makes a sentence clumsier.

"Antibacterial" and "eco-friendly" are usually unsupported. The same FTC actions covered these claims. The original plant has properties the processed viscose does not necessarily retain, and saying otherwise needs evidence most sellers do not have.

Check the blend percentage. A set that is 95% viscose from bamboo with 5% spandex behaves like bamboo viscose with some stretch. A set that is 60% polyester with a bamboo mention on the front behaves like polyester.

Check where it ships from. Not a fabric question, but a returns question. Sizing on sleepwear is a coin flip, and an exchange that involves an international parcel is an exchange you will not bother with.

The short version

If you sleep in	The problem is
Polyester or fleece	Traps heat, holds sweat against you, does both deliberately
Cotton	Breathes, but soaks through and stays wet for hours
Bamboo viscose	Breathes and dries; wrinkles, and costs more

If you are changing one thing tonight and spending nothing, take the polyester out of the bed. If you are changing one thing and spending something, change the layer that touches your skin first, and the sheets second.

That is the order we would do it in, and we sell both, so read that with whatever salt you think it deserves.