All mattresses are not created equal.

People spend about one-third of their lives asleep, and most of this time is spent on a mattress. It’s important to give some thought and attention to the type of mattress you use—the right one can ensure a good night’s sleep and well-rested feeling the following day; the wrong one can cause sleeplessness, back pain and overall aches. When selecting a mattress, make your decision based on firmness, type and size.

Don’t assume soft and fluffy is best. Poor support can lead to muscle stiffness and neck and back pain. Make sure your mattress isn’t too soft and doesn’t contain bumps, valleys or depressions. Conversely, a mattress that’s too stiff can put pressure on your shoulders and hips. The ideal surface is gently supportive and firm, not rock hard or squishy. The mattress should mold to your body while supporting it.

Keep in mind that mattresses don’t last forever. Over time, they lose their firmness and support. The average life of a mattress is 10 years, although most people keep them much longer. Once your mattress has developed lumps and sags, it’s time to replace it.

Polyurethane foam. These come in different degrees of firmness, but they often make people hot while sleeping. When a mattress doesn’t allow air to circulate, it can make you feel hot and sweaty.

Innerspring. These mattresses consist of rows of tempered steel coils layered between insulation and padding. Firmness and durability is based on the thickness of the wire and the number of coils. The higher the coil count, the firmer the mattress.

Waterbeds. These don’t breathe and tend to sag under your body’s heaviest parts. Before you buy one, sleep on someone else’s to see if it meets your expectations.

As a rule, bigger is better. If you sleep with someone, you don’t want to fight for space every night or get kicked, elbowed or shoved on a regular basis. A healthy sleeper moves around 15 to 30 times during the night, and cramped conditions can make sleeping awkward, uncomfortable and altogether frustrating. Some research even suggests that sleeping in the same bed as someone else is less restful than sleeping alone. Also, as you and your bed partner get older, your sleep will become more restless and you may require extra room.

Regardless of which mattress you buy, always try it out in the store before making a final decision. Salespeople expect you to lie on their beds as part of your decision-making process. Assume your normal sleeping position and stay there for a while to determine how it feels. If you have a bed partner, have them join you on the mattress. Even better, ask if the mattress comes with a trial period that allows you to exchange or return it if it’s not right for you.