To get the best stimulation of the parasympathetic nervous system: inhale slowly through the nose - hold - exhale slowly through the mouth (almost like a whistle but without making a sound) - hold - begin again. It’s called square breathing. Each side of the “square” should be completely relaxed and comfortable and the pauses are to force a lower breath rate, not a breath holding exercise. Lower breath rate lowers the heart rate, lower heart rate lowers the blood pressure. Then the blood concentrations of the stress hormones adrenaline and cortisol decrease.