A woman checking a bed to see if she should flip a foam mattress.

Most foam mattresses are made up of several layers, each with different types and densities of foam, says Chris Regan, who oversees CR's mattress testing.

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What's more, each manu­facturer tends to layer foam in a distinct order, which gives it a defined top and bottom. And, of course, many innerspring and a few foam mattresses come with a "pillow top" of soft, cushy material that's meant to stay on top.

All of which is to say that if you flip a foam mattress, you could degrade your sleeping comfort—so always check the manual for advice first. For example, the Serta Hotel Beds Concierge Suite II, an innerspring with a layer of gel-infused foam, can be flipped. But a Casper can't. Instead, you may be able to rotate your mattress from head to toe—except for mattresses that have a specialty zone, such as a firmer lumbar area that should not be repositioned.

Though flipping and rotating may help extend the life of some mattresses, there's no need to rule out mattresses designed to stay put: CR's durability testing has found that most models, whether foam or innerspring, don't sag much, even after we simulate eight years of aging.

Editor's Note: This article also appeared in the November 2018 issue of Consumer Reports magazine.