Start with a count of four in and four out, then drop the count. Once the rhythm is there your body will choose its own length: six and six, eight and eight, whatever settles you.
The count is scaffolding. Holding onto it too long turns the practice into something you can pass or fail, which is the opposite of the point. Handing the rhythm back to yourself also means you notice when it stops being comfortable, and can adjust before anything gets forced.
Qur’an and Sunnah
by the dawn that softly breathes (Quran 81:18). Dawn is described as breathing gently and without effort, which is close to the quality you are after here.
Translation: M.A.S. Abdel Haleem, The Qur'an, Oxford University Press, 2004.
What the research says
A meta-analysis of 24 studies of heart rate variability biofeedback, which trains people towards a slow steady breathing pace, found large improvements in stress and anxiety from before to after, and a more moderate advantage over control conditions. The gap between those two numbers is worth holding onto, since some of what people feel comes from sitting down and paying attention at all. The pace still appears to be doing something of its own.
Goessl VC, Curtiss JE, Hofmann SG. (2017). The effect of heart rate variability biofeedback training on stress and anxiety: a meta-analysis.. Psychological medicine doi
Strength of evidence: one line of research. A single study, or a theoretical account.
Why it works
An even, unhurried rhythm gives the body a steady pattern to settle into instead of the ragged one stress produces.
This one is still a draft. It is waiting on a scholar and a clinician to read it before it is published.
Near this one
Close your right nostril with your thumb and breathe in through the left. Swap fingers at the top, breathe out through the right, in through the right, then out through the left. That is one round.
The fast forceful practices are not for everyone. With high blood pressure, heart trouble or glaucoma, leave them out and use the slow ones, which carry no such warning.
Put a hand just below your ribs and let it move. Most of us breathe up in the chest, and the settling effect comes from lower down, where the diaphragm can actually shift.