In Much Ado About Nothing, the musician Balthazar describes how women sigh when they’re frustrated with men’s infidelity – even though this is a waste of time:

Sigh no more, ladies, sigh no more,
Men were deceivers ever,
One foot in sea, and one on shore,
To one thing constant never.
Then sigh not so, but let them go,
And be you blithe and bonny,
Converting all your sounds of woe
Into hey nonny nonny.

William Shakespeare wrote this just before 1600, but his conceptualisation closely matches today’s understanding of the psychology of sighs. Experimental research shows that, when people complete a stressful mathematics test, look at upsetting pictures, or imagine a past emotion, they sigh more often. This emotion can be negative, such as feeling scared, anxious or depressed, but can also be an arousing positive emotion, such as joy or desire. People also sigh when they feel emotional in their day-to-day life: music students, for example, sigh more when they are anxious before a public performance.

In part, we sigh when we’re emotional because it acts as a ‘reset button’ for our bodies. Scientifically, a sigh is defined as a deep breath – or more specifically, a deep inhalation. Starting in the early 1960s, research in rodents, and later in humans, demonstrated that a sigh serves an essential function by critically protecting the lungs. If people just breathed with the same lung volume all the time, then the alveoli – tiny air sacs in the lungs – would deflate. This would make the lungs stiff, and hinder the essential exchange of oxygen and carbon dioxide. A deep inhalation every so often therefore fully inflates the alveoli, making the lungs flexible again and restoring gas exchange.

This ‘resetting’ regulates our breathing and keeps us healthy – but it also has a knock-on benefit for our emotional state. When we’re stressed, muscle tension gradually increases and our breathing becomes irregular. These changes can be counteracted by a sigh: sighs stretch the respiratory muscles, reduce muscle tension in the body, reduce breathing irregularity, and restore oxygen and carbon dioxide levels when they become too low or high. In this way, sighs reset us physiologically, which leads to a feeling of relief.

The common saying ‘a sigh of relief’ therefore has scientific merit. A study in rats demonstrated that, when they stop receiving electric shocks, their sighing increases. Another found a similar effect in humans: when participants were no longer exposed to loud and stressful noise, they sighed more. Sighs also increase when breathlessness comes to an end, after it’s been temporarily induced in an experimental setting. Importantly, research to date suggests that sighing doesn’t just happen after feeling relieved: sighing leads to relief. It seems therefore that we sigh when we’re emotional because it resets our breathing, reduces muscle tension and induces relief, which in turn helps us manage our feelings.

As a sigh appears to do a lot of good, you might think that intentionally taking a deep breath is always a good idea. But you might want to rethink that. The resetting effects of sighs described above mostly apply to spontaneous sighs – the ones elicited automatically by our physiological or emotional state. Taking a deep breath on demand doesn’t have the same effect. For example, in one study, we asked participants to take a deep breath immediately after a stressful mathematics task, and also recorded any spontaneous sighs they took. We found that taking deep breaths on demand actually inhibited stress recovery in the body rather than facilitating it – this type of breath doesn’t reduce muscle tension like a spontaneous sigh does, and so doesn’t induce the same physiological relief.

On the other hand, in another version of the task, we asked participants to take a deep breath after the stressful maths task but only when they felt like it suited them. In this case, deep breathing helped – it did seem to bring some psychological relief. This is supported by other research: one study, for example, found that taking a series of deep breaths reduced negative emotions and craving in smokers during withdrawal from cigarettes. In sum, if your yoga teacher, best friend or motivational coffee cup says: ‘Take a deep breath,’ and you think: ‘Sounds good, my body could use one,’ then go all in. However, if you think: ‘Not now,’ then that’s perfectly normal and you’re fine to just skip it.

If you find you’re sighing too often, though, this can become a problem. Too much of anything is bad. Although an occasional spontaneous sigh is essential for a healthy lung state and helps us cope with stress and emotions, these beneficial effects can disappear when sighing becomes excessive. People diagnosed with chronic anxiety, panic disorder and post-traumatic stress disorder often sigh excessively; very frequent sighing is also associated with depressive symptoms in people with rheumatoid arthritis. This might be because of the short-term relieving effects of a sigh. If someone learns – often unconsciously – that sighing brings emotional relief, then that behaviour becomes reinforced, and they will do it more often.

As long as a person keeps a healthy balance of oxygen and carbon dioxide, this is okay. But when people sigh excessively, they risk hyperventilating, and thus having chronically low levels of carbon dioxide in the body. This state of ‘hypocapnia’ causes widespread symptoms in the body, such as lightheadedness, palpitations, feelings of anxiety, breathlessness, and pain. In this way, the short-term relieving effects of a sigh sometimes encourage more and more sighing, which can end up perpetuating the very symptoms that people are trying to relieve.

Many scientific questions about sighing remain unsolved. For example, anecdotally, it seems that people who sigh often are mostly unaware of doing so. Most of the time, they are made aware of their frequent sighing by someone else, such as a partner or colleagues. It’s an open question whether sighs in these social contexts have a communication function: are they signals of distress, attempts to express to someone else how we feel, even without us realising it?

It will also be important to understand whether excessive sighing, and other people noticing it, might be useful predictors of chronic disease or psychological distress. If this were the case, it might be possible to train people to sigh less frequently – restoring their breathing and psychology to healthier levels – but, at the moment, we don’t know. When it comes to the mysteries of sighs, only the tip of the iceberg has been uncovered. The 18th-century writer Antoine Bret said: ‘The first sigh of love is the last of wisdom.’ Let’s hope that the love for sigh science will improve the wisdom of sighs in the years to come.