Phase change material

Phase change material, often shortened to PCM, is a coating or fiber treatment that absorbs heat energy as it shifts between physical states, similar in principle to how ice absorbs heat while melting. Applied to a pillowcase or sheet, it creates a noticeably cool sensation the instant you touch it.

That cooling sensation is finite. Once the material has absorbed as much heat as its state change allows, it needs to release that heat and reset, usually by cooling down again during the day or with airflow, before it can feel cold again. This is why PCM products are often described as cool to the touch rather than cooling all night.

The frequent misunderstanding is expecting a PCM pillowcase or sheet to actively pull heat away for hours the way a fan or moisture-wicking fabric does. It is better suited to helping someone fall asleep faster on a warm night, and works best combined with breathable bedding underneath rather than as a stand-alone fix for a hot bedroom.