Almost everything in the book rides on the exhale, so it is fair to ask whether the exhale matters or whether it is just slowness in disguise. Two studies separated the two.
In the first, thirty people breathed at either six or twelve breaths a minute, with the inhale either much shorter than the exhale or much longer. The ratio did more than the rate: a short inhale and long exhale raised reported relaxation, stress reduction, mindfulness and positive energy at both paces, and increased high-frequency heart-rate variability, but only at the slow pace. Slowing down on its own mainly raised "positive energy." In the second, 47 participants tried four patterns; 5.5 breaths a minute with an equal in-and-out ratio produced the highest heart-rate-variability numbers, and all four slow patterns increased felt relaxation over baseline.
So the two studies disagree on which ratio maximizes the heart measure and agree that slow breathing of any ratio feels calmer. A later study in older adults, below under slow breathing and aging, used the low inhale-to-exhale ratio and found it raised vagal activity more in people over sixty than in the young. The book's exhale-weighted breath is the practical version; it is craft in the specific counts and science in the direction.
The studies
Van Diest I, Verstappen K, Aubert AE, Widjaja D, Vansteenwegen D, Vlemincx E. Inhalation/exhalation ratio modulates the effect of slow breathing on heart rate variability and relaxation. Applied Psychophysiology and Biofeedback 39(3–4), 2014, 171–180. Read the abstract
- Sample
- 30
- Design
- Within-subject lab study of four patterns: 6 or 12 breaths a minute, inhale/exhale ratio 0.42 or 2.33.
- Found
- Low ratio (longer exhale) increased relaxation, stress reduction, mindfulness and positive energy at both paces and raised high-frequency HRV at the slow pace; slower rate alone mainly raised positive energy.
- Limits
- Healthy volunteers; single session; self-report plus HRV.
Lin IM, Tai LY, Fan SY. Breathing at a rate of 5.5 breaths per minute with equal inhalation-to-exhalation ratio increases heart rate variability. International Journal of Psychophysiology 91(3), 2014, 206–211. Read the abstract
- Sample
- 47
- Design
- Within-subject comparison of four slow patterns.
- Found
- 5.5 breaths a minute with a 5:5 ratio gave the highest HRV indices; all four patterns increased felt relaxation.
- Limits
- Single session; HRV measured, not clinical outcomes.
Kromenacker BW, Sanova AA, Marcus FI, Allen JJB, Lane RD. Vagal mediation of low-frequency heart rate variability during slow yogic breathing. Psychosomatic Medicine 80(6), 2018, 581–587. Read the abstract
- Sample
- 6
- Design
- Pharmacological blockade study: slow breathing at eleven rates under sympathetic blockade, vagal blockade, or placebo.
- Found
- Vagal blockade nearly eliminated the heart-rate oscillation during slow breathing; sympathetic blockade left it unchanged. The slow-breathing HRV signal is almost entirely vagal.
- Limits
- Six people. A mechanism study, included because it answers a question the field argued about for years.
Where this lands in the book
Techniques
Rooms
Two rules, no exceptions
Never practice any technique on this site while you are driving, operating machinery, or doing anything that needs your full attention. The slow ones can make you drowsy, faster than you’d expect. And never practice a breath-hold, bellows breath, or any vigorous breathing in or near water. Read Before you begin for the doctor’s list and the rest.
Verdict scale: Science means trials or meta-analyses with consistent findings; Promising means real trials that are small, few, or unreplicated; Mixed means the studies point both ways; Mechanism means it shows how something works in the body, not that it helps in life. Checked against the published abstracts in October 2026. Not medical advice.