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Stress happens. And that's not always bad—starting a new job or getting married can both be happy things, but they also can be really stressful.

There are some kinds of stress that just don't seem to go away, though. Like the feeling that you're drowning in work, but still perpetually worried about making ends meet. If you deal with a lot of stress every day, for months or years on end, then stress doesn't just feel awful—it actually causes you physical harm.

Psychologists call any event or situation that puts pressure on you or threatens your well-being a stressor, while stress refers to your psychological and physical reactions. Stressors that are one and done—like locking your keys in your car, or forgetting your wallet—bring on acute stress. But when stressors are repeated or continuous, that's chronic stress.

Things like abusive relationships, living in poverty, and being discriminated against have all been shown to cause chronic stress. And that psychological anguish takes a toll physically. When you experience acute stress, your body activates a system called the hypothalamic-pituitary-adrenocortical axis, or just HPA axis because why would you want to say all that other stuff over and over again.

It starts deep in your brain, in the limbic system — the part responsible for a lot of your automatic emotional reactions, among other things. There, a region called the hypothalamus releases hormones that start a whole chain of more hormones being released — first by your pituitary gland, and then by your adrenal glands, which release a bunch of adrenaline and cortisol into your bloodstream. And those two hormones trigger the “fight-or-flight” response.

They boost physical activity by increasing your blood sugar and the blood flow to your muscles, and bump up your metabolism at the same time. The idea is that the physical boost helps you fight the stressor or run away. So, like, if you were suddenly face to face with a bear, the surge in energy would help you either outrun it, or go all like Revenant on it.

The same system is activated by chronic stress, but things get a bit more complicated. Researchers have found that people under some kinds of chronic stress have perpetually high cortisol levels, as if their HPA axis is running constantly. For others, it can depend on the timing, with higher cortisol levels near the start of the stress before it actually dips lower than usual.

But we do know that while this stress reaction can be helpful at times, having it running all the time is a problem. People under chronic stress are at higher risk for all kinds of ailments, like heart disease, autoimmune diseases, and mental disorders like anxiety and depression. That's because, in addition to it being super unpleasant to be stressed out all the time, the stress response is constantly sapping your energy.

The resources used by fight-or-flight have to come from somewhere, and one of the places they come from is your immune system. On the molecular level, the same cortisol that works to get extra glucose to your muscles also stops your body from making as many infection-fighting white blood cells as it normally would. So stress can tank your ability to fight infections.

It's kind of like evolution is telling your body not to worry about fighting off that cold right now, because you need to fight that bear that is right in front of you. Except with chronic stress, the bear isn't a bear. It's your crappy job.

Or your unhappy relationship. Or whatever it is that stresses you out all the time. And that means your immune system never gets the chance to recover and deal with that cold as easily as it normally would.

One famous experiment demonstrating this involved 11 dental students who volunteered to have their mouths biopsied twice: first during summer vacation, and then again during exam week. It took an average of 3 days longer for the wounds to heal while they were stressed about exams. All kinds of other studies have gotten similar results — some by punching small holes in people like they did with the dental students, and others by observing how stress affects recovery from surgery and other major wounds.

There's also research suggesting that chronic stress explains part of the relationship between poverty and health. Even just the perception of being in a lower socioeconomic class is associated with an increase in respiratory infections. Stress can also advance the aging process.

By the time you get older, your DNA has had to replicate so many times that the protective parts at each of the ends of the chromosome, called telomeres, can kind of start to fray. When telomeres are shorter, it's more likely that there will be errors in copying genes. And those errors increase your risk of disease.

There's evidence that having more cortisol in your blood interrupts the repair of telomeres. Which might explain why stress is linked to diseases that are also associated with age, like heart disease, cancer, and anemia. To stay healthy, the best thing you can do is get rid of the chronic stress.

But, easier said than done. If you can't get rid of it completely, things like meditation and relaxation therapies can help lower your stress response. And, weirdly enough, so might changing how you think about stress.

Studies have shown that when people think about the source of stress as a challenge to overcome instead of a threat to their well-being, that seems to lower their perceived stress and reduce their body's physical response. There's another way you might be able to improve your health, too: help others reduce their stress. In a sample of over 800 older adults, those with high stress who also reported helping friends or neighbors with things like housework or childcare had mortality rates similar to those with low stress.

Whereas those with high stress who didn't help out had reduced odds of survival. So, chronic stress is not good for anyone. But even if you can't avoid being stressed out all the time, there are ways to help yourself relax — and sometimes you can even reduce other people's stress in the process.

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