I think the fact that things like exercise, breath work, meditation and the others work is more a fact of the body being a interrelated system (where nearly everything impacts something else) which we don't fully understand (or, let's face it, we barely understand, as a such).

Just take the impact of the gut microbiome as an example - we are just discovering now how much of an impact that has on a wide variety of other systems in our body. (& so, you could add 'food as medicine' to the list of things that make a measurable difference).

On a related note, have you read the studies being done on plants and anaesthesia? Plants have no 'brain' nor a 'nervous system' in the way that mammals do, and they are 'slow' by comparison to mammals, but they've done experiments with Venus Fly Traps (one relatively 'fast' responding plant) and found that using the same anaesthetic as is used on humans, the plant also stops responding to stimulus. So, what is anaesthetic acting on in the plant?

All very interesting questions!