Breath: Review and Summary Notes
Breath: The New Science of a Lost Art by James Nestor
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Most of this book is pseudoscientific but I think the main conclusion to breath primarily through your nose and not your mouth is intuitive advice, and having mouth tape (using surgical tape) at night while sleeping helps keep you from being dehydrated.
The optimum amount of air we should take in at rest per minute is 5.5 liters. The optimum breathing rate is about 5.5 breaths per minute. That’s 5.5-second inhales and 5.5-second exhales. This is the perfect breath.