Short answer: Breathability is how easily air and water vapour pass through a fabric. It is decided mainly by how the fabric is constructed, which means the knit structure, how tightly it is knitted, how much it weighs per square metre, and how fine the individual filaments are. It is not decided by which fibre the fabric is made from. In a laboratory it is measured as air permeability, under the standard ASTM D737.
Every section below starts with the plain version, then goes deeper for anyone who wants the science. Read only the cream boxes and you will still get the whole picture.
What does breathable actually mean?
In plain termsBreathable means air can get through the cloth, so heat and damp escape instead of sitting against you. It does not mean the fabric deals with sweat.
The deeper dive
In textile terms, breathability covers two related things: air moving through the fabric, and water vapour escaping through it. The two travel together, because vapour rides on moving air, which is why air permeability is used as the practical measure of both.
Breathability is about air and vapour. It is not about liquid sweat. A fabric can be extremely breathable and still leave you lying in a wet patch, because moving liquid is a separate job done by a separate mechanism.
How is breathability measured?
In plain termsA machine blows air at the fabric and counts how much gets through. More air through means more breathable.
The deeper dive
The standard test is ASTM D737, Standard Test Method for Air Permeability of Textile Fabrics. A machine clamps the fabric, pulls air through a fixed area at a fixed pressure, and reports how much air passes. Results appear either in litres per square metre per second, or in cubic feet per minute, depending on the lab. A higher number means more air passes through.
What actually controls breathability?
In plain termsHow the cloth is made matters far more than what it is made of. Lighter and looser lets more air through. Heavier and tighter does not.
The deeper dive
Four construction variables do most of the work.
| Lever | What it does |
|---|---|
| Knit or weave structure | Determines how many gaps the fabric has and how open they are. An interlock knit and a single jersey of the same yarn behave differently. |
| Tightness, or stitch length | Shorter stitches pull loops tight and close the gaps. Longer stitches open them. |
| Weight per square metre | More material in the same area means less open space. This is why a heavy fleece and a light jersey of identical fibre are not comparable. |
| Filament fineness | Very fine filaments pack differently from coarse ones, changing both the gaps and the surface area available for vapour to move across. |
Finishing matters too. Coatings, softeners and calendering can all close a fabric down after it is knitted.
The scale of the construction effect is much larger than most people expect. In published textile research, woven chef jacket fabrics measure somewhere around 60 litres per square metre per second, and hospital operating room uniforms somewhere in the 40 to 50 range, while lightweight single jersey knits have been reported anywhere from roughly 370 to 1390 depending on fibre content, thickness and weight.
Read that last figure again. Within one fabric type, single jersey, the range is close to fourfold. No fibre choice produces a swing that size. Construction does.
How 12 sleepwear fibres actually behave
In plain termsHere is every fibre your pyjamas are likely to be made of, and what each one does with sweat and air. Nothing wins everything.
The deeper dive
This is our own reference scorecard, built from published textile research. Ratings are fibre tendencies, all else equal. They rank inherent leanings under comparable construction, and they cannot be read off a specific garment without testing that fabric.
Excellent Good Moderate Limited Poor Varies
| Fibre | Wicks | Absorbs | Dries | Air flow | Vapour | Cool touch | Softness |
|---|---|---|---|---|---|---|---|
| Kottinu engineered polyester microfiber, 159 gsm |
Excellent | Poor | Excellent | Varies | Good | Good | Good* |
| Polyester (standard) | Varies | Poor | Excellent | Varies | Varies | Moderate | Varies |
| Cotton (combed) | Limited | Good | Poor | Varies | Good | Good | Good |
| Viscose from bamboo | Limited | Excellent | Poor | Varies | Moderate | Good | Excellent |
| Viscose / rayon | Limited | Excellent | Poor | Varies | Good | Good | Excellent |
| Modal | Moderate | Good | Poor | Varies | Good | Good | Excellent |
| Lyocell / TENCEL | Moderate | Good | Moderate | Varies | Good | Good | Excellent |
| Eucalyptus rayon | Moderate | Good | Moderate | Varies | Good | Good | Excellent |
| Merino wool | Limited | Excellent | Poor | Varies | Good | Poor | Varies |
| Silk (mulberry) | Limited | Good | Moderate | Varies | Good | Good | Excellent |
| Nylon / polyamide | Moderate | Limited | Good | Varies | Moderate | Good | Good |
| Hemp | Limited | Good | Poor | Varies | Good | Good | Limited |
* Softness of Kottinu is our own assessment of the finished fabric rather than a fibre-level tendency, because hand-feel depends on the knit and the finish rather than the polymer. We rate it comparable to cotton, and not as soft as modal.
Notice that the Air flow column says Varies for every single fibre.
That is not a gap in the data. It is the finding. Air permeability is set by how a fabric is built, not by which fibre it is made from, so no fibre can carry an air flow rating on its own. Any brand telling you their fibre is inherently breathable, or that yours is inherently not, is making a claim the science does not support in either direction.
The answer is always a specific fabric. Ours is a fine filament engineered knit at 159 g/m², which is a light construction, and that weight is a large part of why it moves air the way it does.
What each column is measured by
| Column | Standard test |
|---|---|
| Wicks | AATCC 197 vertical wicking, AATCC 195 moisture management |
| Absorbs | Moisture regain |
| Dries | AATCC 201 drying rate |
| Air flow | ASTM D737 air permeability |
| Vapour | Water vapour transmission |
One rule worth carrying away: never pool numbers across these tests. They measure different things on different scales and cannot be averaged or ranked against each other. A single blended score across all of them is not a measurement, it is an opinion.
How to read the table
In plain termsFabrics that soak sweat up are slow to dry. Fabrics that dry fast do not soak anything up. You are choosing between those two, not looking for one that does both.
The deeper dive
No fibre wins every column, and any table that showed one would be wrong. Look at the pattern instead.
The absorbent fibres, viscose, viscose from bamboo and merino, rate Excellent at soaking sweat up and Poor at drying. That is one property, not two. The engineered synthetics invert it: Poor at absorbing, Excellent at drying. Cotton sits in the middle and dries slowly. The lyocell family is the most balanced and leads nothing.
Kottinu leads on wicking and drying and rates Poor on absorbency. That is the design, not a shortcoming. A fibre that holds very little water is a fibre that gives it up fast, which is the whole point when you sweat through your nightclothes. On softness it sits with cotton rather than with modal, which is the honest place for it. If you only ever run slightly damp, one of the absorbent fibres may genuinely suit you better, and we would rather say that than sell you the wrong thing.
Is polyester breathable?
In plain termsSometimes. A cheap heavy polo shirt sits hot. A light, finely knitted polyester does not. Same fibre, completely different cloth.
The deeper dive
The advice you will read online, that polyester traps heat and does not breathe, is a fair description of most polyester most people own. Standard apparel polyester is often heavy, tightly knitted, made from coarse filaments and finished for cost. That fabric genuinely does sit hot. If your reference point is a supermarket polo shirt, your instinct about that garment is correct.
What does not follow is the jump from that garment to the polymer. Polyester fibre has no air permeability value of its own, because air permeability is a property of a fabric, not of a fibre. Knit the same polyester open, light and from very fine filaments and you get a completely different result.
So the useful question is never which fibre. It is how was this specific fabric constructed, and can the brand tell you.
Is cotton breathable? Is viscose from bamboo breathable?
In plain termsSame answer. Denim and a summer blouse are both cotton. Nobody would call them the same thing.
The deeper dive
Cotton flannel and cotton voile are both cotton, and only one of them is something you would wear in a heatwave. Fabrics labelled viscose from bamboo, which is chemically viscose rayon, are usually knitted soft and drapey, and that construction is what people are responding to when they describe them as breathable.
The fibre is doing less of the work than the marketing suggests, whichever fibre it is.
Breathability, wicking and drying are three different things
In plain termsLetting air through, pulling sweat away and drying off are three separate jobs. A fabric can be brilliant at one and useless at another.
The deeper dive
| Property | What it does | Standard test |
|---|---|---|
| Breathability | Lets air and water vapour pass through | ASTM D737, air permeability |
| Wicking | Moves liquid sweat along and through the fabric | AATCC 197 vertical wicking, AATCC 195 moisture management |
| Drying | Releases the moisture back into the air | AATCC 201 drying rate |
| Absorption | Takes water vapour into the fibre itself | Moisture regain |
Which one do you actually need at night?
In plain termsStuffy but dry, you want air flow. Actually sweating, you want the sweat moved and dried. They are not the same purchase.
The deeper dive
| Your night | The property that matters |
|---|---|
| Warm room, stuffy, not really sweating | Breathability |
| Sweating through your nightclothes | Wicking and spread |
| Intense episodes, then trying to resettle | Drying rate |
| Slightly damp and clammy | Vapour absorption |
Most people shopping for cooling sleepwear are in row two or three and are being sold row one, because breathable is the easiest word to say.
How to judge a fabric when you cannot test it
In plain termsAsk what it is made of and how heavy it is. Hold it up to the light. Light and open beats heavy and dense.
The deeper dive
- Ask for the fibre content and the fabric weight in grams per square metre. A brand that gives you neither is asking for trust it has not earned.
- Hold it to the light. Open structure shows.
- Weight in the hand is a reasonable proxy. Lighter usually breathes better within the same structure.
- Be sceptical of breathability claimed on the strength of the fibre name alone, in either direction.
Where our fabrics sit
Our NightCool collection is made from Kottinu, a technical synthetic first developed for firefighter base layers and medical scrubs, now made for hot sleepers and night sweats. It is an engineered polyester microfiber knitted at 159 g/m², a light construction from very fine filaments, and that build is where its air flow and its wicking both come from. Yes, it is polyester. We would rather say so and give you the fabric weight than hide the fibre behind a trade name.
Our DayBasics collection is soft everyday sleepwear. We do not market it as a cooling line and it is not built for wicking.
There is a men's NightCool collection too, because men get night sweats and the category mostly behaves as though they do not.
Common questions
Is polyester breathable?
It can be, and often is not. Breathability is set by the knit structure, tightness, weight and filament fineness of the specific fabric, not by the polymer. Heavy tightly knitted polyester breathes poorly. Light open polyester made from fine filaments breathes well.
What is the most breathable fabric for sleeping?
There is no single answer at fibre level, because construction decides it. In our fibre scorecard, air flow is rated Varies for all 12 fibres for exactly that reason. A light, open, low weight knit will out-breathe a heavy dense knit of any fibre.
How is breathability measured?
As air permeability, under ASTM D737, reported in litres per square metre per second or in cubic feet per minute. Higher means more air passes through.
Does breathable mean it will keep me dry?
No. Breathability moves air and vapour. Keeping you dry when you sweat properly depends on wicking and drying rate, which are separate properties with separate tests.
Is natural always more breathable than synthetic?
No. Denim and flannel are natural and not breathable. A fine open synthetic knit can pass more air than a dense cotton one. The comparison only means something when construction is held constant.
Which fibre dries fastest?
Engineered polyester rates Excellent for drying because it absorbs almost no water into the fibre. Viscose, viscose from bamboo and merino rate Poor, because the same absorbency that soaks sweat up is what makes them slow to release it.
Is wicking fabric soft?
It depends on the knit and the finish rather than the fibre. Our own fabric rates comparable to cotton for softness, below modal and viscose, which are the softest fibres in the table.
If you want the next layer down, read why moisture-wicking pajamas still leave some people soaked, and how to read a moisture-wicking claim.
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