Why we sigh

What a sigh is

To a physiologist, a sigh is a double-sized breath. The most cited study on the subject, published in Nature in 2016 by Li and colleagues, describes it as a two-phase breath with about twice the volume of a normal one: a second inhale stacks on top of the first before any air goes out.

It happens by itself, and often. The same study mentions several an hour in humans and dozens an hour in rodents. A 2022 review puts it this way: normal breaths repeat every few seconds, sighs every few minutes.

What it does for your lungs

Your lungs end in tiny sacs called alveoli, where air and blood trade gases. During quiet breathing, some of them gradually close. The 2022 review, by Severs, Vlemincx and Ramirez, explains that sighing helps control how stretchy the lung stays precisely because it prevents that collapse: the extra inhale pops them back open.

The same review notes that low oxygen increases sighing, and that this ties the sigh to the arousal response. It's a maintenance reflex, not a flourish.

The circuit that triggers it

The 2016 study went looking for where the command comes from, in mice and rats. It found two small groups of neurons in a breathing center of the brainstem: about 200 on each side that make a peptide called neuromedin B, and about 30 that make another, gastrin-releasing peptide.

Those neurons send their signals to the preBötzinger complex, the generator of the breathing rhythm. There, about 200 neurons carry the receptors for those peptides. When the peptides were injected at that spot, anesthetized rats sighed 6 to 24 times more. When both receptors were blocked, sighing nearly disappeared. And when the neurons carrying them were destroyed, baseline sighing dropped by about 95 percent.

These are rodent experiments. Nobody has done the equivalent in people, and the authors leave it as a hypothesis for future work that the circuit also takes in emotional signals.

Sighs and emotions

Elke Vlemincx has spent years measuring sighs in the lab. In three experiments published in 2009, her team created brief moments of stress with a loud noise, or the wait for one, and moments of relief when the noise ended or people learned it wasn't coming. People sighed more during relief than during stress.

A 2010 study looked at what happens around each spontaneous sigh. Beforehand, muscle tension was climbing and breathing was getting more disorderly. Afterward, tension fell and breathing settled back into a more organized pattern.

That's the source of the idea the group argues for in its 2022 review: a sigh works as a reset. It marks the shift from one state to another, in breathing and in emotion alike. It's a proposal well supported by small experiments, not a demonstrated mechanism.

Sighing on purpose isn't the same thing

Here the evidence splits. In the 2010 study, a sigh done on instruction didn't bring the same effect: muscle tension dropped less, and recovery from stress seemed to stall. In a 2016 study, people did report more relief after an instructed deep breath. Physical tension, though, fell after spontaneous sighs, and only in people who were more sensitive to anxiety.

The 2023 Stanford trial tested something else: repeating the gesture for five minutes a day, for 28 days. It enrolled 108 adults, 30 of them in the cyclic sighing group. Positive mood rose more with the breathing exercises than with meditation, and cyclic sighing stood out the most. In-the-moment anxiety fell equally in every group. Its authors present it as an exploratory study, with small groups and unsupervised practice.

When a lot of sighing deserves attention

More isn't better. The 2022 review proposes that occasional sighs help regulation, and that excessive sighing does the opposite. The 2009 study points out that frequent sighing has also been linked to panic and pain.

If you notice you're sighing all the time with a feeling that you can't get enough air, or it comes with dizziness, chest pain or palpitations, talk to a health professional. A breathing exercise won't settle that, and there may be causes worth checking.

How to use it

Cyclic sighing repeats the gesture with a count: breathe in through your nose for 2 seconds, breathe in again for 1 second, and breathe out through your mouth for 6 seconds, for five minutes. After a scare uses just three sighs and follows with a minute of slow breathing. If you'd rather leave the inhale alone, Long exhale 4-8 and the One-minute pause keep only the letting-go part.

Keep the inhales gentle. Filling your lungs to the top many times in a row can make you dizzy: if it does, stop and breathe normally. And leave your spontaneous sighs alone. They know what they're doing.