Pick a pattern. Or let one pick you.

Box breathing is four equal phases: in for four, hold for four, out for four, hold for four. The symmetry is the point. There's no climb and no drop, just a square wave.

Inhale four. Hold seven. Exhale eight. The exhale is twice the inhale, which is where the work happens. A longer exhale shifts the balance toward parasympathetic activity, slowing heart rate and lowering arousal.

Five seconds in, five seconds out, no holds. That's six breaths a minute, which sits in the resonance frequency band where heart rate, breath, and blood pressure synchronise into a single oscillation.

Six in, six out, no holds. A slightly slower cousin of coherent breathing. Five breaths a minute, the lower edge of the resonance band.

Inhale four, exhale six, no holds. The exhale is longer than the inhale, which is where the calm comes from. A short asymmetric wave you can run in bed without thinking.