Interesting topic! One I prescribe to as my outdoors activities are the best stress reduction technique I have ever found. The physical component is a factor but not the exclusive component. Sound, smell, visual component....
Unfortunately I cannot find the exact study but many years ago they did a study of retirement home residents and found the mere presence of plants improved cognition.
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4419447/
https://www.scientificamerican.com/a...e-you-smarter/
I may be biased if there is such a thing but my compadres and I have often noted how relaxing the sound of natural water flow is... I've camped near "noisy" rivers/streams and yet slept like a champ. Invigorating sleep is my best description.
I swear insomniacs should give it a try....having occasionally battled the same it has been my go to reset.
Our amateur interpretation is that we're hardwired to the frequencies or what they refer to as auditory texture.
Cheers, Runner