Memory foam is polyurethane foam – which consists of millions of spherical shaped, open cells – with additional chemicals added to increase density. In regard to feel and comfort, what makes memory foam unique from polyurethane foam is its temperature sensitivity and weight sensitivity.
Being temperature sensitive, memory foam tends to be firmer (more viscous) in cool temperatures and softer (more elastic) in warm temperatures.
Keep in mind, however, that low density memory foam is often less temperature sensitive than high density memory foam. As a result, low density memory foam retains a more consistent feel and firmness regardless of temperature. (Learn more about high- vs low-density memory foam.)
Temperature sensitive also means that memory foam – especially high density memory foam – responds to body heat by molding to the curves of a person's body within minutes. This characteristic explains why people often experience a "melting in" feeling when lying on memory foam.
This molding or conforming ability may make moving on or getting up off the bed somewhat difficult. And for some high-density memory foam especially, this molding ability can for some people give the sensation of sleeping in mud or quicksand.
Memory foam is also weight sensitive. Unlike polyurethane foam that compresses and returns to its original shape right away, memory foam returns more slowly to its original shape, thereby having longer "memory" than polyurethane foam. High-density memory foam tends to have a slower response time than low-density foam.
People who don't weigh much (under 120 lbs or so) often say that they don't sink into memory foam. Due to this, they can't fully benefit from the conforming ability of the foam and as a result tend to experience excessive pressure points. This problem can occur with all densities, but it is especially an issue with higher density foam.
Memory foam was originally developed by NASA to lower the extreme physical pressure put on astronauts when they experience high g-force.
The foam was never used by NASA in practice. However, Tempur-Pedic developed a modified version of the foam called Tempur® and it was used as mattress material in the medical industry. The foam helped immobile patients find better comfort and improved health.
Eventually, Tempur-Pedic began selling their product to the non-medical consumer market.
Soon other foam makers began offering their own versions of memory foam, often at a lower density and price. Memory foam is now used not just for mattresses, but for pillows, footwear, chairs and more.