NOT: "THAT WHICH DOESN'T KILL YOU..."
BUT: "JUST ENOUGH STRESS..."
CLOSE TO "KILLING" YOU?
I never could quite "get" (understand) how it was a soothing or helpful phrase to say "what doesn't kill you, will strengthen you". That seems a bit extreme, like it might hurt, and that it might encourage stupid behavior. It also feels "hollow", devoid of reality.
It doesn't kill you, but it can damage you, though the human body can take big jolts and tend to recover from the worst. I would be careful about overchallenging yourself and expecting yourself to be super-human or beyond the normal expecations of a healthy human.
JUST ENOUGH, "JUST RIGHT"
Yes, "just enough stress" will actually slightly challenge your body so that it does build, metaphorically, "stress muscles", so that you have more resilience and essentially a "stress inoculation". And put that stress inoculation together with the practice of using stress emergency tool when stress is too high (exercise right away, deep breathing, which are both very, very doable...and, with a little practice, relaxation).
UNDERSTAND THE PROCESS AND WHAT STRESS IS
But normal stressing of the body and the mind has beneficial effects. Everything one does requires some stress or there would be no action.
Scientists define stress as "a physical, mental, or emotional factor that causes bodily or mental tension".
It's not stress we are to avoid, but stress to too great a degree and/or for too long a duration. My body recovers if given sufficient time, but if stress is continued beyond the degree that the body can handle it without causing malfunctions somewhere else in the body then it will "kill you" - perhaps a little bit at a time, but it definitely will kill you. (It's like voluntary murder.)
Talking about workers who get a small amount of radiation, but an overall true statement for stress in general: "In limited doses, it causes brain cells to overcompensate and thus gird themselves against future demands. Scientists call this phenomenon stress inoculation." (John J. Ratey, MD in "Spark")
Within the reasonable range of stress, the cell cleans up its own waste products that are produced, such as free radicals. It produce enzymes that "mop up waste". These "internal antioxidants" are powerful such that you would have to eat more broccoli than you could eat to get the same antioxidant effect.
The cascade of repair chemicals that are produced are the "growth factors brain =-derived neurotrophic factor, IGF-1, fibroblast growth factor, and vascular endothelial growth factor." The stress process also cause glutamate to increase the production of antioxidants in the cell as well as "protective proteins". And it stimulates brain cell growth, strengthening the brain against stress. It ramps up growth factors, further supporting the neurons in the brain.
Yes, these are all evolutionary reactions to "damage", in order to repair the body, but they also strengthen the body (and mind) if they are in the right doses of "just enough stress".
HOW DO I KNOW I AM BELOW THE THRESHOLD OF TOO MUCH?
To some extent, we simply are stuck with estimating it intuitively at the time. You can "feel" if you're too stressed.
There are signs where you find yourself feeling exhausted (super strong sign) or noticed that you are hyperventilating (with rapid breathing) and tensing and holding your body partially or overall. And you "don't feel good".
Yes, we could be off in estimating the exact amount of stress, but our objective is not to risk by trying to get to the outer limits of where we should be to be able to give the body the ability to recover. That would be foolish.
We simply "stay within a moderate range", feeling a slight pressure or a slight buzz or a bit of extra energy or anxiety for a bit, but you handle it.
You can be assured that you are doing the right stressing by doing your learning in 60-90 minute blocks and then recovering for 10 minutes. See Living In 90 Minute Segments. (An example of too little stress, which will cause you to get mentally and physically flabby is "vegging out in front of the TV" [for more than 30 minutes, at which point mild depression starts])
