The exercise was simple: inhale for a count of five, then exhale for a count of five. Do that for 20 minutes, twice a day, for four weeks. Volunteers' heart rate variability increased during each exercise period and the levels of amyloid-beta peptides circulating in their blood decreased over the four weeks of the experiment. That's because the way we breathe affects our heart rate, which in turn affects our nervous system and the way our brain produces proteins and clears them away. A 2020 study found that heart rate variability drops on average 80 percent between twenty and sixty...