If the litmus test for a dead pillow is that it doesn't bounce back when folded in half, then my pillows are undoubtedly flat-lined. Come to find out, this could be costing me some valuable sleep.

"We stress the importance of having an appropriate sleeping environment, and that includes a comfortable pillow and mattress," says Dr. Ramadevi Gourineni, a neurologist with Northwestern Memorial's Sleep Disorder Center.

Some tips on what to look for when shopping for a new pillow:

1. Get in line. Top priority for all pillows is to keep the head and neck properly aligned with the spine.

2. The position of comfort. Achieving proper alignment is all about density, and density depends on the position in which you most often sleep. Side sleepers require a firm pillow. Back sleepers need a medium-density pillow, and stomach sleepers, a soft, relatively flat pillow.

3. Fill it up. Once you've met your alignment needs, it's time to consider fill. "There are two types of fill: synthetic and natural," says Lauren Somrak, basic bedding buyer for Macy's. "Synthetic fills, which are essentially polyester, are naturally hypoallergenic and a bit more firm than natural. Natural fills are typically either feather or down and are a little softer." Go with the fill that best meets your density and allergy needs.

4. Speaking of allergies. "Pillows have to be replaced or cleaned: Dirt and spores accumulate, increasing allergic symptoms, which can hinder sleep," Dr. Gourineni says. Synthetic pillows are naturally hypoallergenic. Natural pillows, on the other hand, must be manually cleaned. Talk to your linen retailer for specifics.

5. Don't be stingy. Synthetic pillows run approximately $10 to $50; natural pillows around $60 to $130. Spending a little more will get you higher-quality materials, but your own care is the biggest factor in a pillow's life. "If you never fluff it or clean it or put a pillow protector on it, it's not going to last very long," Ms. Somrak says.