Cool Sleep Glossary: 22 Terms Explained
The words that show up in cool sleep specs, reviews and how-to guides, explained without jargon. Open any term for the full explanation and what to look for when buying.
- Bamboo viscose
- A soft, silky fabric made by dissolving bamboo pulp into rayon fibers, valued for its cool feel though less breathable than linen.
- Read more about bamboo viscose
- Bed cooling system
- A pad with circulating water or air controlled by a bedside unit, the most direct way to set a specific sleeping-surface temperature.
- Read more about bed cooling systemRelated: Best Bed Cooling Systems
- Breathability
- How easily air and moisture pass through a fabric or mattress layer, a key factor in whether bedding sleeps cool or traps heat.
- Read more about breathability
- BTU
- British Thermal Units, the unit used to rate how much heat a portable air conditioner or fan-based cooler can remove per hour.
- Read more about btu
- Buckwheat hull pillow
- A pillow filled with roasted buckwheat hulls that lets air circulate between the grains, trapping far less heat than solid foam.
- Read more about buckwheat hull pillow
- Cross-ventilation
- Airflow created by opening or fanning opposite sides of a room, which cools a bedroom far more effectively than a single fan alone.
- Read more about cross-ventilation
- Down alternative
- Synthetic fill, usually polyester, designed to mimic the loft of down feathers without holding as much body heat.
- Read more about down alternative
- Dual-zone cooling
- Bedding or a cooling system split into two independently controlled sides, letting partners set different sleep temperatures.
- Read more about dual-zone cooling
- Duvet insert
- The filled interior layer of a duvet, sold separately from its cover, whose fill type and TOG rating largely decide its warmth.
- Read more about duvet insert
- Gel-infused foam
- Memory foam with cooling gel beads or swirls mixed in to reduce heat build-up, though the effect is often modest and short-lived.
- Read more about gel-infused foamRelated: Best Cooling Gel Mattress Toppers for Hot Sleepers
- GSM
- Grams per square meter, a measure of fabric weight and density. Lower GSM blankets and sheets generally sleep cooler.
- Read more about gsm
- Hot sleeper
- Someone who routinely wakes up feeling overheated or sweaty at night, often needing breathable bedding rather than just a colder room.
- Read more about hot sleeper
- Linen
- A flax fiber fabric that breathes very well and softens with washing, a popular choice for summer bedding and hot climates.
- Read more about linenRelated: Best Linen Sheets for Summer
- Lyocell (Tencel)
- A fiber made from wood pulp, often eucalyptus, spun into a smooth, breathable fabric that is also good at managing moisture.
- Read more about lyocell (tencel)Related: Best Eucalyptus Lyocell Sheets
- Mattress topper
- A removable layer placed on top of a mattress that changes its feel and, with the right materials, how warm or cool it sleeps.
- Read more about mattress topperRelated: Best Cooling Mattress Toppers
- Moisture wicking
- A fabric's ability to pull sweat away from skin and spread it across the surface so it can evaporate instead of soaking in.
- Read more about moisture wickingRelated: Best Moisture-Wicking Sheets for Hot Sleepers (2026)
- Percale
- A plain one-over-one cotton weave that feels crisp and cool, often preferred by hot sleepers over the silkier sateen weave.
- Read more about percaleRelated: Best Organic Cotton Cooling Sheets
- Phase change material
- A fabric coating that absorbs heat as it changes state, creating a cool-to-the-touch feel the moment skin makes contact.
- Read more about phase change material
- Sateen
- A weave with more threads floating on the fabric surface, giving sheets a silky feel and soft sheen but usually a warmer sleep than percale.
- Read more about sateen
- Silk pillowcase
- A pillowcase woven from mulberry silk that feels cool to the touch and glides against skin, though it does not actively wick sweat.
- Read more about silk pillowcase
- Thread count
- The number of threads woven per square inch of fabric. Past a moderate level it says little about coolness on its own.
- Read more about thread count
- TOG rating
- A measure of a duvet or blanket's warmth and insulation. Lower TOG numbers mean lighter, cooler bedding for warm sleepers.
- Read more about tog rating