Sleep science · Thermal regulation
Heat is the enemy of deep sleep.
Your body has to shed heat to fall asleep, and stay out of the heat to sleep deeply. Everything you wear to bed either helps that happen or gets in the way.
01 · The problem
Most people sleep too hot. It shows in how they sleep.
0%
of adults sleep too hot at least some of the time.
0%
sleep too hot always or most nights.
0%
of regular hot sleepers rate their sleep fair or poor.
Gallup and Casper, State of Sleep survey, February 2023. 3,962 US adults, Gallup Panel.
02 · Too hot vs too cold
Running hot costs you more than running cold.
Same survey, split by how people sleep. Hot sleepers were far more likely to report a poor month of sleep than people who rarely overheat. For cold sleepers the gap was smaller.
Sleep researchers see the same pattern in the lab: with normal bedding on, heat cuts deep and REM sleep, while mild cold leaves sleep stages largely intact.
Heat
Cold
Source: Gallup and Casper, February 2023, n = 3,962.
03 · The zone
Get into the zone. Stay there all night.
Under the duvet you sleep inside your own small climate. When sweat stays at your skin, that climate turns warm and damp, and you drift out of the zone where deep sleep happens.
Illustration of the principle. Not an overnight measurement.
Cotton. Holds sweat on the skin side, so the air next to you heats up until you half wake, throw off the covers and cool down. Then it builds again.
SERIS™. Moves sweat to the outer face, where it spreads out and can evaporate, so the air at your skin stays drier.
04 · The moisture test
One drop of sweat. Two fabrics. 120 seconds.
SGS dropped test liquid onto the skin side of each fabric and timed how long it took to reach the outside. Press play. The first seconds run in real time, then the clock fast-forwards to the end of the test.
AATCC 195 moisture management test, SGS reports 223606 to 223608. Outer face wetting time: SERIS™ Glacier Grey 9.44 s, Jet Black 9.35 s. Cotton control: 100% cotton jersey, 180 gsm, outer face not wetted within the 120 s test. Animation timed to the reported results.
05 · The scorecard
Independent lab. Head to head with cotton.
Moisture Management Fabric. SGS classification for both SERIS™ colourways. The cotton control did not qualify.
ONE-WAY TRANSPORT
Above zero, sweat moves away from you. Below zero, it stays at your skin.
STRETCH
Extension across the width under the same 2 kg load. It moves when you turn over, and recovers its shape better too: 85.8% against 80.8%.
AIRFLOW*
+16%
Air passing through the fabric, measured at a fixed pressure.
Figures for SERIS™ Glacier Grey unless stated. Jet Black: transport index +291 (grade 4), extension 184%. Moisture to AATCC 195 (SGS 223606 to 223608). Stretch and recovery to BS EN 14704-1, width at 2.0 kg (SGS 223606i to 223608i). *Air permeability to BS EN ISO 9237, Glacier Grey only (SGS 224485, 224484). Cotton control: 100% cotton jersey, 180 gsm.
06 · How SERIS™ works
Built into the fibre. Nothing sprayed on.
SERIS™ is 93% MicroModal Air and 7% elastane. At 0.8 dtex the fibre is extremely fine, so the yarn carries a large surface area for moisture to travel along. The moisture handling comes from the fibre itself, with no chemical finish applied.
01
Pick it up
The skin side takes in sweat as it forms.
02
Move it out
It travels one way through the fabric, toward the outer face.
03
Spread it
On the outside it spreads thin over a wider area.
04
Let it go
Spread out, away from your skin, it can evaporate.
07 · The bottom line
Cool down to fall asleep. Stay dry to stay asleep.
You can't control every degree in your bedroom. You can control the layer on your skin for eight hours a night.
08 · The research
Read the studies yourself.
Sleep temperature and sleep qualityGallup and Casper, State of Sleep survey, 2023
Web survey of 3,962 US adults on the Gallup Panel, 10 to 17 February 2023. 57% said they sleep too hot at least occasionally, 14% always or most of the time. 46% of typically hot sleepers rated their sleep fair or poor, against 29% of people who rarely sleep hot.
Effects of thermal environment on sleepOkamoto-Mizuno and Mizuno, Journal of Physiological Anthropology, 2012, 31:14
Review of human sleep studies. With bedding and clothing on, heat exposure increased wakefulness and reduced slow-wave and REM sleep. Humid heat added to the thermal load. Cold with bedding left sleep stages unchanged.
The temperature dependence of sleepHarding, Franks and Wisden, Frontiers in Neuroscience, 2019, 13:336
Sleep onset and a fall in core body temperature happen together. Falling asleep is most likely when core temperature is dropping fastest, and warming the skin speeds up sleep onset.
The thermophysiological cascade leading to sleepKräuchi, Sleep Medicine Reviews, 2007, 11(6):439-451
In the evening, blood vessels in the hands and feet open and release heat. This heat loss drives the fall in core temperature that comes before sleep.