I noticed that the Honeydew Side Sleeper Pillow felt very cool to the touch. That's because the filling is made with CertiPUR-US-certified copper-infused foam. The website says copper is far more effective at cooling than gel-based memory foam and much cozier. I don't care why it's cool — I just care that it is! I didn't realise how much warmer my regular pillow was until I had this cooler pillow to compare it to.