Here's what's going on when your nips stand at attention.

You probably don't spend too much time thinking about your nipples—until they demand your attention. Maybe that's when you're working out sans shirt and all of a sudden it's like there's a pair of headlights poking out from your sports bra. Or maybe every time the temperature drops below a certain degree, you feel like they could cut glass.

But what actually causes your nipples to get hard? Just like no two women have the exact same breast shape, no one's nipples respond the same to different scenarios. But, anatomically, there is a general reason for why they occasionally stand to attention.

The Science of Why Nipples Get Hard

Anatomy time: "Underneath the nipple and areola (the area surrounding the nipple), there are tiny muscles that contract and pull on the skin and push the nipple out," says Michelle Lee, M.D., a plastic surgeon based in Beverly Hills, CA. In response to any kind of stimulation, "the sympathetic nervous system—part of the body's neural wiring that makes your heart race, skin get goosebumps, and palms sweat—sends signals to the nerves in those tiny little muscles that cause them to contract," explains Dr. Lee.

Of course, that stimulation might be sexual. You probably already know your nipples are a sensitive erogenous zone. Arousal—whether it's from physical stimulation (e.g. touch) or psychological stimulation (e.g. fantasizing about something 🔥)—triggers certain parts of your brain that "cause those muscles in your nipples to become erect, similar to the effect those aroused feelings can have on your genitals," says Heather Irobunda, M.D., an ob-gyn based in Queens, NY. In fact, one study found that nipple stimulation in women activated the genital sensory cortex, the same part of the brain turned on by stimulation of the clitoris, vagina, and cervix.

And just like a clitoris or penis, nipples are comprised of erectile tissue, a type of tissue that receives a ton of blood flow, says Dr. Irobunda. "When the genital sensory cortex is activated, that blood flow can cause that tissue to get harder or stiffer," she adds. (BTW, it is possible to have a nipple orgasm—here are nipple play tips to get you started.)

But stimulation isn't just about arousal. If your nipples get hard as rocks when the AC is on high, that's totally normal. "When the temperature drops, those tiny muscles underneath the skin contract to trap the warm air near the skin and lose less heat from radiation coming off your skin," says Dr.Lee. "This is why the areola also contracts when the nipple is exposed to the cold. The contracted skin then pushes the nipple outward."

Sometimes, it's not even about stimulation. Hormones can also make your nipples hard when there's no other obvious reason. "During your period or even ovulation, changes in your hormone levels—especially estrogen—can cause your nipples to become more sensitive or even be more prone to become harder," says Dr. Irobunda.

Other hormone-related nipple changes start in pregnancy and continue through breastfeeding. "During pregnancy, the hormone progesterone causes the formation of more milk lobules and duct systems," explains Dr. Lee. And during the late stages of pregnancy, many other hormones cause the areola to get larger and the nipple to get erect in preparation for breastfeeding, she adds. "During breastfeeding, mechanical stimulation of the nipple from the baby causes the body to release oxytocin; this hormone acts on the milk glands to contract and push milk out to the baby," says Dr. Lee.

Do nipple piercings alter how your nipples get hard?

If you've ever seen someone with a nipple ring, their erect nipple is probably pretty obvious. Besides the fact that placing a large piercing at the base of the nipple will physically push it out, "having jewelry there causes increased sensitivity due to the constant stimulus to the nerves in the nipples," says Dr. Irobunda. A nipple piercing doesn't mean that your nipples will be hard 100 percent of the time, but it can certainly make them more noticeable and, because they're more sensitive, may become hard more easily.

If You're Concerned About Having Hard (or Not Hard) Nipples

Whatever your nipples do, there's probably no reason to be concerned. "Nipples come in all shapes and sizes, and depending on the size and position of the nipple, some nipples may appear to be never hard or always hard," says Dr. Lee. "How sensitive your nipples are to stimulus is genetically predetermined." (Note: Nipples that never get hard are commonly referred to as flat or inverted nipples.)

If you're self-conscious to the point where you don't want to take your shirt off in a workout class or at the beach, you may want to talk to your doctor.

On a more serious note, "if the nipple suddenly becomes retracted or inverted, or if you notice nipple discharge, dimpling, pain, or lump, it's important to see a doctor and figure out the reason why," says Dr. Lee. (And here's what you should know about itchy nipples.)

And "if your nipples are not only hard but painful for an extended period of time (versus during certain parts of your cycle), I would recommend going to see your doctor," says Dr. Irobunda. "This may be indicative of a larger health problem that needs to be addressed." (Related: 11 Signs of Breast Cancer Every Woman Should Know)