Maybe you've heard, but we spend a full one-third of our lives asleep. "A mattress is so important since it's the one thing that is closer to us than anything else when we are sleeping, during that one-third of our lifetime," says Dr. Neil Kline, a board certified sleep physician and a spokesperson for the American Sleep Association. "During the day, we can [consciously] adjust our positions and change our behavior, but at night we cannot. We really depend on our mattress," says Bart Haex, a professor of biomechanics at the University of Leuven in Belgium and author of Back and Bed: Ergonomic Aspects of Sleeping. Here's what you need to know to be sure you can count on yours to give you the best night's sleep possible.