Why Do We Sigh?

You just sighed. You probably didn’t notice.

On average, you sigh roughly once every five minutes — about 12 times per hour, over 200 times per day. Most of these happen without your awareness. You’re reading, working, having a conversation, and somewhere in the background your body quietly doubles the depth of a single breath and carries on.

For centuries, sighing was understood as an emotional behavior. A sigh of relief. A sigh of longing. The sigh as literary shorthand for sadness, yearning, exhaustion. Leonardo da Vinci believed sighing served to cool the heart when distress had heated it.

The actual function is more mechanical, and stranger than any of these.

The Alveoli Problem

Your lungs contain roughly 500 million alveoli — tiny air sacs where gas exchange between blood and air actually occurs. These sacs are elastic, kept open by a layer of surfactant (a lipid coating that reduces surface tension at the air-liquid boundary in the sac wall).

Under normal breathing, the alveoli don’t fully inflate with each breath. Tidal breathing — the normal in-and-out at rest — moves only about half a liter of air, which is enough to exchange gases but not enough to keep every alveolus fully open. Some alveoli partially deflate and stick together. Their walls touch, the surfactant film collapses across the junction, and they become difficult to re-inflate.

A sigh — a breath approximately twice the normal volume — provides enough pressure and volume to force these stuck alveoli back open. This resets the mechanical state of the lungs, prevents progressive alveolar collapse, and maintains efficient gas exchange.

Without sighing, alveoli would progressively collapse. The clinical term for this is atelectasis, and it’s a genuine medical problem — it occurs in patients who are mechanically ventilated without a sigh function included, and it impairs oxygenation significantly. When ventilator engineers discovered this, they deliberately built periodic forced deep breaths into mechanical ventilators. They had reinvented the sigh.

The Neural Control

In 2016, researchers at Stanford and Caltech identified the specific neurons that control sighing — a small population of cells in the brainstem’s pre-Bötzinger complex that add a second breath on top of a normal breath.

These neurons fire periodically, independent of emotion, independent of conscious choice. They simply run the maintenance program: every few minutes, double one breath. The cells responsible for detecting the need (alveolar deflation pressure) and the cells responsible for executing the response (the sigh motor pattern) form a dedicated, automatic circuit.

This is the physiological sigh: a built-in lung maintenance tool that runs without you.

What’s interesting is that you can also trigger it voluntarily. Taking a deliberate deep breath, even a double-inhale, activates the same mechanical result — alveoli re-inflate, gas exchange improves. Breath control practices across many cultures (the deliberate deep breath to manage anxiety, the yogic full breath) may be exploiting this maintenance mechanism for purposes other than the one it was built for.

The Emotional Sigh

But sighs clearly do accompany emotion. And the emotional sigh is physiologically distinct from the resting maintenance sigh.

Emotional sighs are associated with relief — the moment of letting go after stress, the transition from effort to rest, the release after holding tension. Neuroimaging studies show that emotional sighs correlate with activity in the medial prefrontal cortex and related emotional regulation regions, not just the brainstem circuits of the resting sigh.

The best current interpretation: emotional sighs use the same hardware (the lung re-inflation mechanism) but trigger it through a different pathway. Under stress, breathing typically becomes shallower and faster — a pattern that compounds alveolar collapse and progressively impairs oxygen exchange, which feeds back into the physiological experience of anxiety. The emotional sigh may represent a reset: forcing a full inflation, slowing the breathing rate, and breaking the shallow-breath-anxiety feedback loop.

If this is right, the sigh of relief is both emotionally genuine and physiologically functional. The feeling of release is real, but part of what’s happening is that your body is literally re-inflating collapsed lung tissue and restoring normal gas exchange.

Why You Can’t Stop Sighing When You’re Sad

One of the frustrating features of sadness is that it seems to produce more sighs, which can feel like visible displays of suffering you’d rather not be making.

The mechanism may be the same one in reverse: emotional distress produces shallow breathing, shallow breathing produces alveolar collapse, and the brainstem sigh circuit responds. You’re not sighing because you’re performing sadness — your body is running respiratory maintenance in conditions that keep requiring it.

There’s also a social component. Sighs are acoustically audible, and people are sensitive to them. A sigh in a conversation carries information — it often marks a transition, a letting-go, a shift in emotional state. Whether this communicative function is incidental to the respiratory one, or has been reinforced by it over evolutionary time, is unclear.

What is clear: the sound of a sigh can be more communicative than words.

The last time you sighed, it was almost certainly automatic — a lung maintenance breath that your brainstem ran without consulting you. Before that, another one. And another.

They kept your alveoli from collapsing, oxygenated your blood a little more efficiently, and passed without your notice.

Twelve times an hour, your body quietly fixes itself.

You never get the credit.