The 1 Reason You Aren't Getting Stronger
But if the stress is just right, it will cause “adaptation,” a response that restores the state of your physiology to what it was before, plus a little more, so that if the stress is applied again your body is ready for it. The stress/adaptation/recovery cycle operates in all living things, and it is an integral part of existence in whatever environment an organism lives.
Your skin adapts to the stress of the sun exposure by becoming dark enough to prevent itself from burning again. That's the only reason it gets dark, and it adapts exactly and specifically to the stress that burned it. Your skin does not “know” that you want it to get darker. It only “knows” what the sun tells it, and the sun only talked to it for 15 minutes.
It can’t get any darker than the 15 minutes makes it, because the 15 minutes is what it is adapting to. If you got darker every time you were exposed to the sun, and the darkness just accumulated, we’d all be dark -- but only the parts exposed to the sun. Most of us would only have tanned faces and arms, because the skin under our clothes received no stress.
Stress can be varied in a couple of ways. It can vary with the total amount of stress, like jogging two miles vs. six miles. This type of stress variation is called volume. Or it can vary in intensity, like jogging two miles in 18 minutes vs. running two miles in ten minutes.
So if you want your skin to get darker, you have to either stay out longer or go to a place with more intense UV in order to give the skin more stress than it has already adapted to.
The widespread failure to comprehend this pivotal aspect of adaptation is why so few people actually understand the concept of training. (We’ll explore the important differences between exercise and training in a future article.)