How hard should I push myself?.

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"But importantly, I wonder whether pushing myself might, in fact, kill me. Constant pressure creates chronic stress, and there’s all sorts of scientific studies that show that chronic stress is really bad for you. It makes you more susceptible to heart disease, it makes it harder to recover from illnesses, it can affect your sleep, and it can even affect your working memory."

"The stress response is built to get us out of danger. If you're an animal in the Serengeti and you're being chased by a lion you really, really want to have a stress response. Being stressed means you’re preparing your muscles to move—a lot. Your heart rate rises and pushes blood to your extremities. Glucose is released into your bloodstream to help run your muscles as fast as possible. 

The stress response pushes certain parts of your body into high gear—but it also turns certain parts of your body off. For example, when you’re stressed digestion is inhibited. What’s the point of wasting energy on digesting food for later when you might not even survive for 10 more minutes? Reproduction is also inhibited. Same reason. 

This is all well and good in the wild—you don't need to reproduce if a lion is eating your face off. But humans, and some more intelligent animals, have evolved the stress response from an unqualified Good Thing into...well, something that might kill you.

What’s different about our stress response? Well, we have the ability to _anticipate_ danger. Other animals have this ability too: it’s a good thing to get stressed seeing the lion all the way across the savannah, instead of only when it mauls your intestines out. But humans have evolved this anticipation ability to extend far beyond other animals. We anticipate bad things months, years, or even decades out. And when we do this, the very same stress response gets turned on—even though there is no immediate danger, and there is no immediate way to avoid it. 

Suddenly, you aren’t just activating the stress response for a few minutes when you’re running for your life. Instead, it’s activated all the time—chronically. And this is where the problems start.

Remember we mentioned earlier that the stress response amps some parts of your body into high gear, and turns off others? If you’re doing that chronically, you start to have problems. Suddenly, your digestive system isn’t just inhibited for a few minutes while you’re escaping danger. It’s chronically inhibited. The same thing happens with your immune system—chronic stress tamps it down, and makes it harder for you to fight off diseases. Stress is bad for your heart too—if you’re pumping blood as if you need to run from something all the time, you’re going to get high blood pressure."