You finally sit down after a long day, and out it comes: a deep, slow sigh. We sigh when we're tired, bored, annoyed, or relieved. But have you ever wondered why our bodies bother with this extra-big breath in the first place?
The answer is that a sigh is two things at once. It's a way humans show emotion, and it's also an automatic breathing reflex controlled by a small group of cells in the brain stem. Sighs are larger than normal breaths, and research in babies suggests they help keep lung tissue working well.
The Feelings Behind a Sigh
Language experts describe a sigh as a deep, audible single breath out through the mouth or nose that humans use to communicate emotion. It often comes from negative feelings like dismay, dissatisfaction, boredom, or a sense that things are hopeless.
But sighs aren't all gloom. A sigh can also come from relief, especially when something bad ends or is avoided. A sigh is often closely related to a gasp. If something shocks you and then turns out to be less alarming than you feared, the air you sucked in with the gasp may come back out as a sigh.
Like a yawn, a sigh is often automatic and unintentional. You might not even notice you've done it until someone asks, "What's wrong?"
A Reflex Run by a Few Brain Cells
Breathing is normally unconscious and automatic, and much of it is controlled by a region in the brain stem called the pre-Bötzinger complex. It was identified in 1991 by neuroscientist Jack Feldman, who now teaches at UCLA, and his colleagues. That same small network produces both your everyday breaths and your sighs.
Sigh breaths come far less often than normal breaths, and they're two or three times bigger. In 2016, Feldman and his collaborators identified a brain chemical that acts in this region to govern sighing. When they added it there in lab animals, the animals began sighing vigorously.
That means sighing isn't only about mood. A few specialized brain cells keep sighs coming on their own, whether you're having a good day or a bad one.
Why the Body Wants Big Breaths
So what's the point of a sigh, physically? Studies show that babies sigh after every 50 to 100 breaths. This seems to improve the mechanical properties of their lung tissue and helps them develop a regular breathing rhythm.
In other words, the occasional extra-deep breath appears to give the lungs a helpful stretch that ordinary breaths don't. If you ever notice unusual shortness of breath or breathing that worries you, talk to your doctor.
We're Not the Only Sighers
Humans aren't the only ones who do it. Behaviors similar to sighing have been seen in dogs, monkeys, and horses. If your dog has ever flopped down on the rug and let out a long, dramatic breath, you've witnessed it yourself.
Here's a fun bit of trivia. The pre-Bötzinger complex got its odd name from a neighboring brain area, the Bötzinger complex, which was named at a 1980 scientific conference in Germany after a bottle of Bötzinger wine served at dinner.
So the next time you let out a big sigh, don't worry that you're being dramatic. Your brain is simply running one of its oldest routines, and your lungs are probably grateful for it.
