Why We Sigh - The Physiological Job That Has Nothing to Do With Being Tired

Someone Comments on Your Sighing

You are turning pages of meeting material and, without noticing it yourself, letting out a long breath. The person in the next seat asks whether you are tired. You had not thought you were, but once it is said you start to feel that you might be.

There is folklore that sighing lets your happiness escape, and in some cultures another person's sigh is received as a display of ill humor. Seen from respiratory physiology, however, a sigh is not a by-product of emotion but a movement built in as a function for keeping the lungs sound.

What Happens Inside the Lungs

The interior of the lung is built as a finely branching set of airways with countless microscopic sacs, the alveoli, attached at the ends. Oxygen and carbon dioxide are exchanged across the walls of those sacs.

An alveolus is a sac made of thin membrane, and its inner surface is moist. Because wet membranes in contact pull toward each other, an alveolus left alone tends to deflate and close. A closed alveolus cannot take part in gas exchange, so if closures accumulate, the efficiency of breathing falls.

This is where the sigh does its work. Drawing in a larger breath than usual raises the pressure inside the lung and pushes open the alveoli that were closing up. Inserting this movement at intervals keeps the usable surface area of the whole lung available.

The instruction to take deep breaths given to patients after surgery exists to supplement this function deliberately. If nothing but shallow breathing continues, parts of the lung stay closed, which works against recovery.

It Happens to Everyone at Regular Intervals

What shows that sighing is built in as a function is the fact that it occurs independently of emotion. Even when not low, a person inserts a breath larger than usual at a rate of roughly once every few minutes. Most of these pass without being noticed at all.

The part of the brain that regulates breathing carries out this movement. The circuit that generates the rhythm of respiration and the circuit that inserts a deep breath exist separately, and the latter working at intervals is what produces sighs. That you cannot hold one back for long even when you try is because the process is managed at a layer below the will, in the territory the Autonomic Nervous System governs.

In other words, most sighs are maintenance work on the body. Receiving another person's sigh as a declaration of displeasure amounts to reading meaning into something that is happening as a function.

Why It Becomes Tied to Emotion

So why does sighing look so tightly bound to feeling?

One reason is that breathing turns shallow in situations of sustained tension. When you are concentrating, when you are tense, when you are in pain, breathing becomes shallow and fast. If shallow breathing continues, more alveoli close, which raises the need for a large breath to reopen them. A sigh arriving after tension is the result of shallow breathing having accumulated during it.

The other reason is that it works as a punctuation mark. A sigh includes a long exhalation, and the exhaling phase works in the direction of settling the body. A sigh at the moment a difficult task ends, or right after bad news arrives, is thought to be functioning as a movement that switches your state. Seen through the Stress Response Cycle, in which the body's side of the reaction does not end on its own just because the cause has gone, the point of inserting such a switch becomes clear.

Because these two overlap, sighing gets observed as something that comes out when things are hard. In terms of order, it is not that emotion produces the sigh; the change in breathing that accompanies the emotion raises the need for one.

How It Differs From a Deep Breath

A deliberate deep breath and a sigh look similar from outside but occupy different positions.

A deep breath is a movement performed by will, and you set its count and its speed. Taking the form of slow in and slow out strengthens the work that settles the body. The procedures collected under the heading of Breathing Techniques make use of this property, and methods that use conscious breathing to quiet anxiety rest on the same principle.

A sigh is an automatic movement, taking the form of a large inhaling phase and a long exhaling one. Since its purpose lies in lung maintenance, any settling effect is secondary.

There is therefore no need to suppress sighing. Suppress it and lung maintenance stalls. If it bothers you in front of others, making the movement smaller or stepping away to do it is response enough.

When It Feels Like Too Much

Sighing itself is normal function, but if the frequency has clearly increased, the background is worth checking.

First, whether shallow breathing has been continuing. Long stretches in one posture, hunched desk work, and chronic tension all make breathing shallow. In that case, adjusting posture and breaks is more coherent than reducing sighs.

Next, whether it comes with breathlessness or a pounding heart. A pattern in which breathing turns shallow and fast, sighs come repeatedly, and numbness in the hands and feet or mild dizziness appears is known as a state of excessive breathing. This state can arise bound up with anxiety, and if it repeats it is a matter to consult a medical professional about.

Also, if there is shortness of breath, chest pain, or disordered breathing during sleep, evaluation on the respiratory or cardiac side becomes necessary. Continuing to treat sighing as a matter of mood means overlooking changes like these on the bodily side.

That said, in most cases a sigh is the sound of the body setting itself right. There is no need to feel apologetic because someone pointed it out. If it does concern you, looking back at why your breathing had turned shallow helps more in practice than holding the sigh in.