What is the physiological sigh?

A double inhale through the nose followed by a long exhale through the mouth. It copies a reflex your body already performs spontaneously every few minutes to reinflate collapsed air sacs in the lungs. The second, smaller inhale reopens regions that had partly closed, which is what allows the long exhale to offload more carbon dioxide. Practised repeatedly for five minutes a day, under the name cyclic sighing, it was the exhale-focused practice the 2023 Stanford trial singled out.