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.

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.

Strength of evidence: one line of research. A single study, or a theoretical account.