The idea of memory foam is incredibly simple. Instead of sleeping on springs, you sleep on very thick foam that gives way where you press on it. Technically, it's called visco-elastic: visco (from viscous) means it moves when you apply a force; elastic means it returns to its original shape when you remove the force. The foam is temperature sensitive, so it tends to sink more as your body temperature warms it up. Very quickly, it forms to your unique contours. If you sleep on your side, the foam sinks in much more at your shoulders and hips so, in theory, your entire body is always perfectly supported and your spine remains straight. Your weight is spread more evenly and the pressure is relieved on places like your shoulders and hips. That's why many people find memory foam mattresses help to reduce the symptoms of problems such as back pain and arthritis. If you change position or turn over, the mattress immediately takes to your new shape. High-density foam mattresses are also claimed to be more hygienic because bed bugs apparently don't like living in the foam. In reality, bugs can lurk in your bedding, on your carpet, on your walls, and in plenty of other places in your bedroom—so this claim is a bit of an exaggeration. It may be true that memory foam beds reduce the problem of bed bugs, but scrupulous hygiene is really the only solution to that particular problem, not just a change in your mattress.