Why does my mattress feel firm at times and soft and responsive other times?

TEMPUR-Material™ responds to your body weight and heat. It contours to your body, automatically adjusting to your every curve. When not in contact with your body, the material returns to its usual shape (which it’s designed to do so it feels like new every time you sleep on it), so it can feel slightly firmer when you first lay down. After you have been on the mattress a while, the material responds to your body heat and starts to feel more conforming and responsive. The material responds to the room temperature, so if your room is very cold, the mattress can feel less responsive and slightly firmer; if your room is hot, the mattress can feel more responsive and softer. No matter how firm or soft the mattress feels, it still offers the support that your body needs to fully relax during sleep.