So THAT'S Why Your Nipples Get Hard When It's Cold

Exposure to cold temperatures can affect the body in a number of ways, like making your teeth chatter or cheeks redden. But have you ever wondered why nippy weather perks up your nipples?

It’s not just a winter phenomenon — it can also happen in warmer times of year, like when the air conditioning is on full blast. So what exactly is occurring in the body when nipples become erect? 

The physiological process that leads to hardened nipples when cold is also what causes goose bumps (known medically as piloerections), dermatologist Dr. Joshua Zeichner told HuffPost.

“This response is beyond our control and is a result of our bodies’ automated reactions to the environment,” said Zeichner, an associate professor of dermatology at The Mount Sinai Hospital in New York City. 

Essentially, a drop in temperature triggers your sympathetic nervous system, which is behind the fight-or-flight response and other involuntary bodily functions. This process activates specialized nerve cells controlling the erectile muscles that cause hard nipples and goose bumps, according to a 2016 study published in the journal Nature Neuroscience.

“When the body feels cold, the sympathetic nervous system contracts tiny arrector pili muscles, which are attached to hair follicles,” explained dermatologist Dr. Aanand Geria of Geria Dermatology in New Jersey. This creates goose bumps and makes the hair on your body stand upright.

“In the case of nipples, it contracts the specialized smooth muscles,” Geria added. “This contraction pulls on the skin, erecting the nipple.”

As for why this occurs, it’s one way that the body tries to retain heat in cold weather. The muscles beneath the skin contract “to trap the warm air near the skin and lose less heat from radiation coming off your skin,” plastic surgeon Dr. Michelle Lee told the outlet Shape.

For mammals, including humans, “regulating core body temperature within a small range is crucial for survival,” Geria explained.

The reflex that causes goose bumps (and nipple erections) in the cold is not so useful for humans today, but would have been quite important for our ancestors. Back when we were a much hairier species, having all of your body hair stand up helped with insulation — not to mention that it could make you look larger to ward off predators.

Nipples can become erect for other reasons unrelated to the cold, such as sexual arousal, tactile stimulation (like when they rub against your shirt), hormonal changes during the menstrual cycle, pregnancy or breastfeeding, and even “emotional responses like fear or excitement,” Geria said.

Most of the time, hard nipples are just a normal bodily response to a number of different stimuli, he noted — but if you’re seeing “persistent or extreme erection unrelated to temperature or excitement,” then it could be indicative of a health problem. Also, if you’re experiencing pain with nipple erections or any type of discharge, it’s worth talking to your health care provider to rule out any underlying issues.