The Physiological Sigh

A physiological sigh is a double inhale through the nose (a short second sniff at the top of a normal inhale) followed by a long exhale. It occurs involuntarily roughly every five minutes during normal breathing as a mechanism to reinflate collapsed alveoli — the tiny air sacs in the lungs tend to deflate during shallow breathing. The physiological sigh is the lung's automatic reset. Researchers at Stanford (Yackle et al., 2017, Science) identified the specific neural circuit — a small cluster of neurons in the pre-Bötzinger complex — that generates this automatic double inhale, which exists in mice and humans.

Anxiety-related breathing tends to be shallow and rapid, which causes CO2 to drop (hypocapnia), which paradoxically increases the sensation of breathlessness and amplifies anxiety in a feedback loop. The physiological sigh's double inhale maximally inflates the lungs, and the long exhale following it stimulates the vagus nerve and slows heart rate more effectively than a single deep breath. A 2023 study by Balban and colleagues (Cell Reports Medicine) found that cyclic sighing — five minutes of deliberate physiological sighs — outperformed mindfulness meditation and box breathing in reducing self-reported anxiety and improving mood across the day.

A single deep inhale through the nose opens alveoli but does not fully reinflate those that have collapsed during previous breathing cycles. The double inhale — a full breath followed by a second short sniff — creates a pressure differential that forces open the more stubborn collapsed alveoli. The result is more complete lung inflation and, on the exhale, a more complete vagal stimulation. Regular deep breathing is effective for relaxation; the physiological sigh is faster-acting for acute anxiety because it addresses the alveolar collapse that is contributing to the breathless sensation.

Yes. The double inhale through the nose is subtle — it looks like a slightly longer breath from outside. The technique is deliberately not the exaggerated, visible breathing of other techniques. You can practise it at a desk, in a meeting, in a conversation, or on a train without it being apparent. The exhale should be slow through the mouth or nose — a slow exhale through the nose is entirely invisible. For situations where you cannot pause at all, even two or three physiological sighs during natural breathing pauses (between sentences in conversation, for example) provide a measurable effect.