No one seems to have mentioned Wim Hoff here ( https://www.wimhofmethod.com/ ). He’s got some claims that verge on the suspicious but the actual method itself is worth doing purely if you’re an interested sort. Before, I can do about 40 seconds held on an out breath, after, about 2:40. That’s kinda interesting. I’d also recommend trying it if you’re a meditator. Do a WH session first, then sit. It’s really great for…
It’s actually an old technique to increase the ability to hold the breath. The hyperventilation clears carbon dioxide from the system (hypocapnia). That’s what the buzz is.
Never ever ever ever ever do this underwater or you have a serious risk of death: https://en.wikipedia.org/wiki/Freediving_blackout#Shallow_wa...
Hof did not warn of this at the time, though I believe he does now after someone died using the technique while swimming.
As for the cold resistance, Hof’s twin brother Andre displayed the same physiological traits that Hof has, despite not having undergone the same training: https://journals.plos.org/plosone/article?id=10.1371/journal...
The one interesting thing I have seen related to Hof is that he apparently was able to suppress the immune response to an endotoxin. Though it would be interesting to see that same study done on his twin.
This is a reasonably good article: https://www.pepijnvanerp.nl/2016/01/wim-hof-method/#andre
I think Hof is well intentioned, sincere, a poor communicator, and prone to both mystical thinking but also interested in scientific verification.