As memory foam warms, it becomes softer. Low-density (3lbs and under) memory foam instantly provides a softer feel beneath you.
Medium-density (4 - 5 lbs) memory foam provides a good mix of a firm to soft ratio, ideal for comfort and support especially for those who do not any medical attention for joint or muscle pain.
High density (6+ lbs) also known as an orthopaedic mattress, recommended for those requiring medical attention for aches and pains.