Cheryl Mendelson, author of Home Comforts: The Art and Science of Keeping House (Scribner), says that the ideal mattress is firm enough to keep your spine aligned, but soft enough to cradle as much of your body as possible. "The more you distribute the weight, the less stress you put on the pressure points," she says. That's why Mendelson loves pillow-top mattresses, which add a layer of cushioning to a moderately firm mattress. (The top layer can also be purchased separately and added to another mattress.)

Though she can't vouch for the ultra-luxes, she certainly wishes she had firsthand experience with them. "If I were a rich woman I might," she says.

Fortunately, Mendelson is quite happy with her own more moderately priced Eastman House pillow-top, available at Thomasville furniture.

Meir Kryger, MD, author of A Woman's Guide to Sleep Disorders (McGraw-Hill) and past president of the American Academy of Sleep Medicine, says that general rules for mattress buying are nearly impossible to come by, since individuals' sense of comfort varies wildly. "In our sleep lab we use standard hospital beds. They're not great. A lot of my patients will say they're unbelievably comfortable; others will say it's the least comfortable bed they've ever slept in," he says.

However, for people with sleep disorders, Dr. Kryger recommends investing in the largest bed you can afford, especially if your (or your spouse's) condition involves a lot of moving or twitching. "This minimizes the effect on the bed partner," he says.