Last updated: Apr 18, 2016

There's nothing quite like sinking into bed after a long day. But if you're literally sinking into your bed, it might be a time for a new mattress, says Richard Guyer, MD, an orthopedic surgeon and cofounder of the Texas Back Institute.

Your spine has a natural S-curve, he explains, with the lower back curving inward, and the upper back curving out: "If you lay down on a very, very soft bed, the curve is not supported and you lay like you’re in a hammock. I call it the banana position.” While the banana position feels oh-so-good when you’re chillaxing, it’s not the way you want to sleep all night, says Dr. Guyer.

So how do you know if your mattress is firm enough? “It’s one of those black box areas that we as consumers don’t know much about," says Dr. Guyer. But there are a few telltale signs that you're not getting the support you need: 1) There's a dent in your mattress in the morning. 2) You're draggy throughout the day, because you're not getting quality sleep at night. 3) You feel soreness in your back in the A.M. (Your too-soft mattress may not necessarily be the cause of the pain, Dr. Guyer points out; but it can certainly make back pain worse.)

If any of these signs are true for you and you're ready to invest in a better mattress, Dr. Guyer has a few shopping tips.

Buy a reputable brand, but skip the top of the line

Mattresses from the priciest brands usually have too much padding, Dr. Guyer explains. Many are sold with a "pillow top" that can be so thick it's almost like having no support at all, he says.

Choose a firmness rating of 4

Brands typically grade their mattresses from 1 to 5, with 1 being the softest and 5 being the firmest. Dr. Guyer suggests that his patients choose a 4, which should be juuust right.

Memory foam is a good option if you're a fan

Mattresses made from this cushy polyurethane material aren't for everyone. But if you like the feel, Dr. Guyer recommends memory foam because it contours to your natural anatomy, allowing an impression of your thoracic spine and lower pelvis while also supporting the rest of your back.