A memory foam mattress pad gives you added comfort and support by completely conforming to your body and relieving pressure points. It can also help prolong the life of your mattress by reducing surface wear.
The "memory" in memory foam comes from the molding that occurs when you lie on it. The foam cells are temperature sensitive and become more elastic and viscous with your body heat, so the cells compress and mold around you to fully support your body.
There are different foam densities available. Higher density foams will have 5 pounds or greater density, while medium density foam is in the 3- to 4-pound range. Foam mattress pads also come in different thicknesses, with most falling between 2 to 4 inches thick. A thicker pad will provide more cushion and molding, but be aware of the need for deep-pocket fitted sheets.
Foam density will determine how soft or firm the mattress pad will be at room temperature. A high-density foam will stay firm and provide more support, while lower densities will be softer and will take slightly longer to return to their original shape after you leave the bed.
According to the manufacturers behind Healthy Foundations Memory Foam Mattresses, some deeply discounted memory foam is very low density and may have sand or other fillers mixed into the foam when it's poured. Avoid extremely inexpensive foam mattress pads if there's any question of quality.