The instant you begin to feel anxious, your body starts to react, Dr. Ross tells Bustle. "The first response to stress begins in the hypothalamus in the brain, which sends signals to the pituitary gland and the adrenal medulla. They start a hormonal cascade," she says. As the cascade spreads, it causes your heart to pound, your breathing to quicken, and sweat to start pouring, all of which are designed to help us cope with threats and danger.

A study published in 2019 in Seminars In Cell & Developmental Biology found that this cascade even affects the microglia, a type of nerve cell in the brain and spinal cord. After danger passes, your body is meant to reduce these hormones to normal levels, but if you're under a lot of pressure all the time, they stay at elevated levels constantly.

Chronic stress can damage your body's defenses against viruses and infections. A review of the effects of strain on the body published in EXCLI Journal in 2017 found that studies have linked stress to poor immune system function, in part because when you're anxious, your body changes the way it secretes hormones that help the immune system. This can lead to something called chronic immune activation, in which your immune system overreacts and starts to attack healthy cells instead of threatening ones.

"Stress leads to systemic inflammation, which can increase chronic pain and impair the immune system, leaving people more vulnerable to infections, from the common cold to flu," Dr. Julia Blank M.D., a family medicine physician also at Providence St. John's Health Center, tells Bustle. A 2019 study published in Microbial Pathogenesis found that it can actually help bacterial growth, making infections worse.

Living under pressure impairs sleep because it makes us alert and panicky, damaging our ability to relax and get refreshing rest. "Chronic anxiety can cause people to experience insomnia," Dr. Blank says. Heightened adrenaline and cortisol levels make it more difficult to calm down, which can stop you from falling asleep or from feeling refreshed when you wake up.

Sleep deprivation can physically alter your brain, making everyday tasks a struggle. A review of the science around anxiety and sleep published in Neuroscience & Biobehavioral Reviews in 2019 found that being stressed just after birth can affect our sleep all the way to adulthood.

It's not just sleep deprivation that makes your brain woolly when you're worried. "For those suffering from constant and on-going stress, long term physical and mental medical complication can occur," Dr. Ross tells Bustle. Feeling worried over long periods can increase the likelihood of depression, anxiety and other mental health issues. Research in 2018 found that anxious people showed slight brain shrinkage compared to relaxed people, and the review in 2017 showed that worry can physically rewire the brain, changing its structure and the way its cells communicate.

If you feel gurgles in your digestive system whenever you're anxious or upset, you're not alone; the digestive system can be very sensitive to emotions. "Stress can cause gastrointestinal changes, including irritable bowel symptoms, diarrhea or constipation, abdominal pain, and nausea," Dr. Blank says.

Research published in Frontiers in Microbiology in 2017 found that being under strain can damage the microbiome that helps your gut function, though the effects can differ widely from person to person.

Life under a lot of strain can put pressure on the heart; when you're worried, your heart pumps harder to distribute blood to make sure you're prepared to deal with threats. "Stress can cause high blood pressure and heartbeat irregularities," Dr. Ross tells Bustle. Being anxious is a risk factor for poorer heart health overall, with stressed people more likely to show symptoms of cardiovascular disease, heart attacks, and other heart issues over the course of their lifetimes. A study published in Circulation in 2019 also found that race plays a role in the relationship between stress and heart health in women, too.

Old wive's tales (not to mention David Lynch's Twin Peaks) often mention people whose hair turned white overnight after a huge scare or shock — and while that might not be common, research published in Nature in 2020 found that stress can directly cause hair-whitening in mice. According to the study, the body's fight-or-flight system negatively impacts melanocyte stem cells, which live in hair follicles and color your hair.

Melanocyte stem cells die as you age anyway, causing gradual whitening over time, but the 2020 study found that stress accelerated the process. Being under pressure can potentially change your hair color, but it's hard to predict exactly how.