HRV

Effects of diaphragmatic deep breathing exercises on prehypertensive or hypertensive adults: A literature review

Slow breathing between 6 to <10 breaths a minute, with or without a device, for at least 10 minutes a day for 4+ weeks significantly reduces blood pressure, improves blood flow to vital organs, and improves autonomic dysfunction in people with hypertension or prehypertension.

Effect of rosary prayer and yoga mantras on autonomic cardiovascular rhythms: comparative study

Recitation of the Ave Maria (Hail Mary) prayer or a common yoga mantra slows breathing rate to 5.5-6 breaths per minute and improves markers of cardiovascular and autonomic health. They can be thought of as both health and religious practices.

Oxygen-induced impairment in arterial function is corrected by slow breathing in patients with type 1 diabetes (2017)

Slow breathing improves autonomic and arterial function while also acting as an antioxidant in people with diabetes.  The last line of their abstract summarizes it perfectly: Slow breathing could be a simple beneficial intervention in diabetes.

Heart rate variability biofeedback in chronic disease management: A systematic review (2021)

HRVB (and, hence, slow breathing) is a powerful adjunctive therapy for managing a wide variety of chronic diseases. To be effective, it should be practiced for a minimum of 10 min/day (but probably 20+ for best results—see next section). More broadly, this study supports using slow breathing at around 5-6 breaths per minute in people with chronic diseases.