"If you can fit three fingers between your lower back and the mattress, then it's not giving enough support," says Karin Mahoney, director of the Better Sleep Council
(the education arm of the International Sleep Products Association). She suggests rotating the mattress 180 degrees every six months. Sleep experts are always hesitant to recommend the perfect model, material or brand of mattress, but medium-firm has been shown in studies and through patient reports to be comfortable for most people, says Delamarter. He says that you'll know it's time to upgrade when you regularly wake up with stiffness, numbness and back pain. He frequently hears of patients who test-drive mattresses at hotel chains before committing to a purchase. Queen-size innersprings are the most popular in the U.S., according to industry data, but the new gel mattresses, launched about a year ago, are said to be the hot new thing.