We bet your mama has been telling you this for years.

PeopleImages/Getty Images

Growing up, we’re taught vanity is bad, and for good reason. But, just once, we’d like to make the case for vanity, at least when it comes to skincare. As your largest organ, your skin works hard to protect the rest of you. And by putting a little extra care into how your skin is looking and feeling, your health benefits exponentially.

With all the advice, it seems there’s a lot you can do wrong—from using products with harsh ingredients to falling asleep with makeup still on. (We’ve all been there.)

But what’s the biggest no-no in skincare? According to Birmingham, Alabama, dermatologist Dr. Julie C. Harper, it’s exposing it to unprotected, ultraviolet light.

“Both natural sunlight and tanning beds emit ultraviolet wavelengths of light that penetrate the skin and damage the DNA and the support structures that keep the skin healthy and looking youthful,” she says. “As we age, the skin on the arms, hands, face and neck show visible signs of aging while the areas that are typically sun-protected, like the abdomen and the buttocks do not show these signs.”

That point we were making about vanity? By protecting the look and feel of your skin, you protect your physical health as well.

"Much of what our culture thinks of as aging skin is all sun-induced aging,” Dr. Harper says. “While I certainly believe in good skin care, NEVER skip sun protection. Use a broad spectrum sunscreen with at least an SPF of 30 on a daily basis on those areas not covered by clothing.”

WATCH: 5 Simple Skincare Swaps to Make When the Seasons Change

The ultimate skincare commandment: lather on the sunblock for youthful, even, and most importantly healthy skin for years to come. And not just when you’re hitting the beach, or on your nose and forehead—like Dr. Harper said, anywhere exposed to sun, every day. What’s one more step in your skincare routine?