The Body Adapts!
I have been using lip balm before going to bed for roughly two years now.
I just came back from a 5-day camping trip with friends. Obviously, I did not bring my lip balm with me and over the course of 4 days, my lips started to get dry and cracked even though I never had this before using lip balm. Presumably, moisture-producing cells in my lips “sense” that moisture levels are already high and stop producing moisturizing secretions whenever I use lip balm. Said mechanism takes a couple of days/weeks to readapt, leading to said effect (reversible).
Relatedly, most people are amazed when I tell them that I never use shampoo or warm water to wash my hair, which on the top is roughly 10cm long. I only ever wash my hair with cold water. My hair does not look greasy at all but has a natural shine to it. When I started doing this three years ago, I had greasy hair for roughly 2-3 weeks. After that period, greasiness went down to “normal” levels. Have not used shampoo since and only use shampoo when my hairdresser applies it when washing my hair.
Similar to my lips, when I wash my hair with shampoo, sebum-producing cells in the scalp sense that sebum levels are lower than they should be, consequently cranking up sebum production (leading to greasy hair after a couple of days of not using shampoo anymore).
Third example. Whenever I wash my hands more frequently with soap (removing endogenously produced oils), my hands start to get sweaty more easily, presumably because moisture/sebum-production is increased.
I find all this fascinating because with each example, there has to be a moisture/sebum-level sensing mechanism somehow turning on an intracellular signaling cascade leading to said effects.
Similar things apply to the central nervous system. For example, in the past, whenever I studied vocabulary, I was soon a much better vocab learner. I expect this to be only partly due to improvements in technique. Presumably, neural circuits related to memorizing vocabulary long-term potentiate. Or whenever I am practicing gratitude, after a couple of weeks, my emotions of gratitude become stronger. In a similar way can a temporary period of fear lead to a full-blown anxiety disorder.
Bones grow in the direction of the pressure applied to them.
People growing up at high altitudes do not just have higher hemoglobin levels, but every level of the organism adapts to the relative hypoxia. From an increased number of oxidative proteins, more abundant mitochondria, and presumably hundreds of other tiny low-oxygen adaptations.
After lots and lots of diving, the spleens of deep divers can “learn” to contract, pumping additional RBCs into the circulation.
Muscles really do grow longer in response to stretching.
Astronauts devoid of gravitational pull develop weird forms of orthostatic dysfunction after landing because their cardiovascular system had adapted to new conditions.
How and that all this evolved is mind-blowing (from a mechanistic level) because in each case specific sensing-mechanisms and a large number of gene expression changes are at play!