Introduction
It happens countless times a day. You’re sitting at your desk, having just dealt with the last email of the day. Or you come home with groceries and have put everything away in its proper place. Or you’ve done your morning exercises and sit down with your first cup of coffee. Something small or significant has been completed.
It happens automatically: an inhalation that’s too big for an ordinary breath, followed by an exhalation that sits somewhere between letting go and landing, accompanied by an audible or inaudible “phew.” As if you’re placing a comma in the sentence of your day.
If you pay attention, you can notice even more changes in precisely that small moment. Your shoulders drop a fraction. Your face relaxes, and your jaw becomes less tense. The eyes soften again and perhaps even your belly. You know where you are again.
Why We Sigh
Sighing is a small but important bodily habit. We sigh approximately every 5 minutes. A sigh is a deeper inhalation (about twice as deep as normal), followed by a deep exhalation.
Physiologically, the sigh has a very practical task. Alveoli can have a tendency to “collapse” slightly; sighing helps to reopen them and keeps the lungs supple and efficient in gas exchange. In the scientific literature, this role is described quite consistently1: sighing supports lung compliance [= the flexibility of the lungs] and prevents (micro)collapse [= the “caving in”] of alveoli [= air sacs].
Additionally, it’s important that both lungs are well ventilated. This isn’t self-evident. Our lungs have some overcapacity; for normal daily functioning, not “all registers need to be open.” But stagnant air, from the alveoli that aren’t directly needed and thus aren’t involved in breathing at that moment, goes stale. With a sedentary lifestyle and cramped shoulders, the upper lung fields will be less well ventilated. With a cramped diaphragm, the lower lung fields can’t expand as far. Oh my, and then to think that there are quite a lot of people with cramped shoulders AND a cramped diaphragm. Our lungs have a hard time and are quite squeezed in many bodies.
So you can consider sighing as lung maintenance.
From Tension to Task, from Task to Letting Go
Besides gymnastics for the lungs, sighing is also useful in stressful situations. Through a spontaneous deep sigh, muscle tension decreases, especially in people who quickly experience anxiety. After the sigh, the dynamics of breathing change. It’s a way of resetting to calm and/or control the breath somewhat more.
For this reason, people also often sigh during transitional moments. For example, just before they begin a difficult task that requires concentration or precisely when that task has been completed. The sigh facilitates the transition from one situation to another so that breathing matches the task.
And Then That “Phew”: The Sigh That Also Becomes a Message
Many people do something extra: they sigh and say, “Phew.” Not an empty sound, but “phew” as an interjection that expresses that you’re glad something is (finally) done, and it’s also linked to relief and fatigue. In the e-ANS (the electronic General Dutch Grammar), “hèhè” is even explicitly mentioned in the list of expressive interjections for relief (alongside, for example, “oef”).
What’s also nice is that language professionals recognize it as something that’s close to a sigh: in the VRT style guide, under the distinction between hé/hè/hey, it states that “hè” is also used “as a sigh,” with the example: “Phew. I’m tired.” And not to forget the 2025 Dutch Book Week essay, written by Paulien Cornelisse, which even has the famous “hèhè” as its title and theme!
So where the sigh in itself can already be a bodily reset, “phew” makes that reset audible and socially readable: it’s (momentarily) over, I’m letting go, I’m landing, I’ve gotten through it.
Sighing and Phew = Breath and Voice = Regulating Indeed!
And so we arrive at a connection with polyvagal theory. A sigh appears to be primarily breathing behavior and lung maintenance. But as soon as “phew” is added, the element of voice is included. The larynx and vocal cords are involved; there’s a micro-expression.
Through the polyvagal lens, important parts of the social engagement system are activated. Here we’re mainly talking about the breath (which has a direct relationship with the vagus nerve, the most essential nerve of the parasympathetic part of the autonomic nervous system) and the tongue, throat and vocal cords, involving not only the vagus nerve but also the glossopharyngeal nerve. The latter is also a significant link in our social engagement system.
But if we pay close attention and if our autonomic nervous system isn’t too chronically dysregulated, we can notice that the effect of the sigh + phew extends further than just the breath and the voice and throat. Many people will notice that the face softens somewhat and that the breath gets more space in the belly. And behold: our entire social engagement system gets a reset and is activated.
The Social Engagement System
As I write in my foundational article on polyvagal theory, besides the well-known and sometimes “hyped” vagus nerve, there are four other cranial nerves of importance in the polyvagal story; together they form our social engagement system. This system is also called the Ventral Vagal Complex. These are evolutionarily relatively new structures that were necessary for mammals to survive. A mammal must, first of all, be capable of establishing a relationship with at least two other mammals, namely a partner and offspring2.
Additionally, it’s of at least equal importance that a newborn mammal (thus also a human baby) has optimal coordination of breathing, swallowing, and sucking; otherwise, feeding would become a life-threatening undertaking! The ventral vagal system plays a major role here.
Finally, a social system provides other advantages that increase our chances of survival; think of the large herds of mammals on the African savanna.
Back to the Sigh
Although the autonomic nervous system is autonomous (duh), which suggests we have no influence over it, we do have several entry points through the ventral vagal system to give ourselves signals of safety and thus influence the evolutionarily older structures of our mobilization and immobilization systems.
The sigh—whether or not accompanied by a “phew”—stimulates the social engagement system, calms stress, relaxes the face, and opens the belly. The nervous system shifts to rest.
I dare to bet that there has never been anyone who was being chased by a tiger and who, halfway through running for their life, let out a sigh, then said “phew” AND survived. Evolution has selected out these naïve souls 😉. You could say that a sigh is the signal that you’re not being chased by a tiger.
Phew, I’m safe!
“Phew” is the way your body says, “I'm home again.”
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See for instance this article.
And of course: birds also have social relationships. So you can assume that birds also have some kind of social engagement system. There are even reptiles that exhibit brood care. Crocodiles are an example of this. The mother guards the nest and helps the young hatch from the eggs. After hatching, she stays with the offspring for some time to protect them from predators. The question, however, is whether you can really call this social behavior, since there’s primarily strong instinctive control of this behavior (which, incidentally, can still look quite loving). Critics see this as “proof” that polyvagal theory is incorrect, because “the social engagement system is apparently not reserved for higher mammals and humans.” That’s too simplistic in my view. It’s a well-known evolutionary principle that “components” are “repurposed” in newer structures*. Porges writes: “The structures that enable us to slow down in safety are structures that were repurposed in mammals from reptiles. These structures are therefore not found in reptiles.” An article about this is coming, stay tuned!
*Participants in my classes and trainings hear me say: “Mother Nature is lazy.” I think that’s actually true. What I mean by this is that Mother Nature wastes nothing; she doesn’t develop new structures if she can reuse old structures; there are countless examples of this. I also use this quote to clarify that unhelpful habits that are still strong were apparently once very useful. Mother Nature naturally wouldn’t let you be very good at something if you never needed it.