In the early 17th century in Venice, Italy, a white limestone bridge was built to connect the Doge's Palace to a prison across the canal. Named the Bridge of Sighs by Lord Byron, it was where people had a final look at the city through the bridge's latticed windows before being imprisoned, and, purportedly, let out sighs of regret or remorse.
Another legend about the Bridge of Sighs is more hopeful: If a couple sails under the bridge and kisses at sunset, their love will last forever, and the act will bring about some lovelorn sighing. (In the 1979 movie A Little Romance, the lead characters, played by Diane Lane and Thelonious Bernard, set out to sanctify their relationship by doing just this.)
Through Bridge of Sighs folklore, the adaptability of the sigh is showcased. Sighs have long been linked in literature, music, films, and in this case, architecture, to emotion. Sighs can accommodate many different feelings: We sigh when we’re happy, relieved, lustful, bored, depressed, or anxious. Perhaps you've recently had reason to let out a big sigh—with the announcement of a promising COVID-19 vaccine or the results of the presidential election. Maybe you'll elicit sighs in others when you explain why you can't travel home for the holidays.
Despite this lived experience we all have that connects sighing to a diversity of emotions, research on sighing has focused largely on the respiratory purpose of sighs, and not how they entangle with our feelings.
But the psychological, or "physiopsychological," aspects of sighing are finally starting to be explored. A recent theory proposes that sighing is not just a reset for the lungs and breathing, but for our emotions too, bringing us back to stasis from big emotions, whether they be positive or negative.
Sighing might also not be just a byproduct of emotions, but could induce feelings too, like relief. Intriguingly, there may also be such a thing as too many sighs. People with anxiety disorders who sigh more than others might be dysregulating their breathing by over-relying on the soothing powers of the sigh. Ultimately, what's being uncovered is that unlike what the song "As Time Goes By" from Casablanca claimed, a sigh is not just a sigh.
A sigh is a very deep breath, defined scientifically as having an inhalation that is at least twice the volume as a normal breath.
Research from the University of Oslo found that most people associated sighs with negative emotions like disappointment, defeat, frustration, boredom, and longing. But the primary physical function of a sigh is for the benefit of your lungs. Sighs keep the tiny air sacs in the lungs, the alveoli, from collapsing, and maintain the exchange of oxygen and carbon dioxide, said Silvia Pagliardini, an associate professor in the Department of Physiology at the University of Alberta.
Healthy adults sigh about once every five minutes. If you don’t sigh and re-open the alveoli, you could become hypoxic and die. People died when using the earliest iron lungs because designers didn’t account for sighing, which modern ventilators now do. When mice are genetically engineered to not be able to sigh, they eventually die of major lung problems.
Historically, the sigh was considered to be like a reflex, Pagliardini said. “The lungs collapse, they send some input to the brain, and the brain makes a sigh," she said. In the past few decades, we've learned that sighs are programmed by the brain to occur no matter what signalling is coming from the lungs.
In 2000, Nino Ramirez, a professor of neurological surgery at the Seattle Children’s Research Institute, and his colleagues published in Nature Neuroscience that a network in the brain stem was responsible for multiple types of breathing patterns, including sighing. In experiments in rats, when scientists sliced the brain stem and put the slices, half a millimeter thick, in Petri dishes, the disembodied brain pieces still showed a respiratory rhythm—including sighs, Ramirez said. No lungs required.
In 2016, Pagliardini and her colleagues were able to understand how sighs are generated in even more detail. In rats, they found a small cluster of neurons in an area of the brainstem called the pre-Bötzinger complex that generates normal breathing as well as sighing and gasping. Specific molecules called neuropeptides activated those brain cells and generated a sigh. “If you add these peptides into this specific part of the brain, you increase the sighs, and if you block the receptors that detect these neuropeptides, you sigh less,” Pagliardini said.
"Sighing appears to be regulated by the fewest number of neurons we have seen linked to a fundamental human behaviour," Pagliardini's co-author, Jack Feldman from the University of California, Los Angeles, said in a press release at the time.
But though we now know the mechanics, Ramirez said we're just at the beginning of understanding how sighing interacts with the brain more generally. In just one example of where this knowledge could be relevant, the pre-Bötzinger complex controls neurons that have norepinephrine in them, a chemical that causes arousal. There is a theory that in infants that die from Sudden Infant Death Syndrome (SIDS), this arousal system doesn’t work. The babies don’t sigh properly, or the connection to the norepinephrine neurons is disturbed, and babies don’t wake up when their breathing is disturbed, leading to their death.
“Physiologists for many centuries only thought about breathing as a mechanism for the lungs to provide you with oxygen,” Ramirez said. “They overlooked the role of breathing as a mechanism to control your brain state.”
Breathing isn’t only the automatic exchange of oxygen and carbon dioxide, afterall. The way we breathe is influenced by our emotions and environment, and vice versa. Think about anxiety, panic, or pain; or, pleasure, a crush, relief—all of these emotions can influence our breathing, and our sighing.
Scientists had mostly ignored how sighs influence behavior and emotion, until Elke Vlemincx —an assistant professor at the Department of Health Sciences at the Vrije Universiteit Amsterdam—came along.
Vlemincx admitted that she's a bit of a “loner” in the field of sighing. She’s been investigating not just the physiology of the sigh, but the psychophysiology—how the physiological properties interact with the psychological.
The ways that our physical sensations interact with our emotions is an evolving area of research, though not a new one. In the 19th century, William James, the "father of American psychology," proposed that our emotions are simply the names we've given to sensations in our body. Following this line of thinking, we experience feeling relief and then sighing, but it could be that the physical sensation we get from the lungs expanding is what we refer to as the emotion of relief.
Research on how well people feel their own hearts beating has shown that our heartbeats not only react to our emotions but can influence them. Similarly, Vlemincx thinks that sighs have a bidirectional link to our emotions—big emotions can trigger sighing, but sighing can bring about emotions, too.
Ramirez said the emotional importance of sighing is built into language, too. When we say we are "inspired," we’re talking about our brain being excited. “You would never say, 'I have a great idea, I have expired,'” he said. “This whole idea of inspiration driving your brain status is extremely important.”
From many subsequent lab experiments on sighing, Vlemincx and her colleagues have developed a hypothesis: Sighs are like a reset button for both your breathing and your emotions.
Our breathing is automatic, yet it also has a lot of built-in variation and flexibility. Our breathing changes a lot, depending on what we’re doing—talking, laughing, running— as well as how we feel, and our environment. When this breathing variability either becomes too low, or starts to become erratic, Vlemincx has found that sighs come in and hit "reset," to bring us back to the “balance between nonrandom and random breathing variability.”
Psychologically, sighing plays a parallel role, “helping to restore calm in the context of stress or emotional arousal, inducing a subjective sense of relief, and may therefore be functional as a mechanism to cope with stress and emotions,” Vlemincx wrote in a review paper on sighs.
In her first studies, Vlemincx focused on the link between sighs and relief. In a 2005 paper, rats were trained to learn that a certain stimulus meant they would not be exposed to a tail shock—when they saw this “safety” cue (an analog for relief) they sighed 20 times more. Vlemincx has now replicated these findings in humans, showing that sighing significantly increases when people experienced a similar safety cue, and that sighs are associated with both self-reported relief and the release of physiological tension.
If a sigh is a kind of reset, both physically and emotionally, it helps explain the duality of sighs, and how they can be associated with negative emotions, stress, and anxiety, while also being paired with positive emotions like relief, joy, or desire. This seems like a paradox, Vlemincx said, until you think about sighs as an emotional reset. They can be present during any big emotional state, whether good or bad; think of the prisoner on the Bridge of Sighs, or the lovers underneath it.
When she records people sitting quietly, not exposed to any stress or emotional triggers, Vlemincx finds that they sigh at an average rate of about once every five minutes. But there’s a big variation in how much a person sighs, she said, that has to do with a person’s personality.
One of the most consistent findings around sighing is that it’s strongly associated with anxiety. When rats are genetically bred to have high anxiety, they sighed more than rats bred for low anxiety. People in pain, people with unpleasant thoughts, and people doing stressful math problems all sigh more. People with anxiety, PTSD, and panic disorder sigh more frequently than those without those conditions.
Many people who sigh a lot are not aware of it, Vlemincx said. “But people in their environments know,” she said. “They’re often told by people they work with, for example, ‘Wow, you sigh a lot.’ They’re not aware until people tell them.”
Vlemincx thinks that when a person is anxious, they’re more likely to continuously be seeking the relief that a sigh can provide. But there’s a point at which sighing can be overused as a coping mechanism. In lab studies, when observing people with panic disorder, it’s been shown that they sigh excessively. Some are sighing so much that their breathing never gets a chance to return back to normal, and they are chronically hyperventilating.
When this happens, hyperventilation isn't just a side effect of panic, but contributing to the anxious feelings, too, through chronically low carbon dioxide levels in the body. When you hyperventilate, and expel a large amount of carbon dioxide, it can cause symptoms like dizziness or numbness. This can lead people to panic more, breathing more deeply, and then feeling worse.
“If you’re anxious, you sigh a lot,” she said. “And then that excessive sigh rate induces hyperventilation, which comes with anxiety, then you're in a vicious cycle of anxiety.”
For people with panic disorder, who can often feel like they’re short of breath, one treatment, capnometry-assisted respiratory training, or CART, is focused on avoiding taking deep breaths. “Especially for people with high anxiety, taking a lot of deep breaths is not a good idea,” Vlemincx said.
This advice contradicts a lot of messaging about breathing that anxious people may receive, as well as certain breath work practices in yoga or meditation. For many people deep breathing will elicit relief and hit the reset button, but pushing the button too many times can turn into a nasty feedback loop, Vlemincx said.
The more negative the emotions are, the more relief and reward a person gets from a sigh, meaning the more frequently they sigh. But the relief doesn’t last very long. So, people can sigh more frequently, in search of that relief again.
“It’s good to take a deep breath now and then,” Vlemincx said. “You have to, it’s healthy for you. But I will say, you have to make sure it doesn’t get to a point where you dysregulate the system and start hyperventilating. But I always say, when a yoga or exercise instructor says, 'Take a deep breath,' only do so when it does indeed make you feel good.”
Trying too hard to modulate your sighing might not even offer the same benefits as sighing on your own. Vlemincx's work has also found that there is a difference between a spontaneous sigh and an instructed sigh—or a sigh on demand, intentionally taking a deep breath. When Vlemincx and her colleagues asked people to take a deep breath intentionally, and measured relief and muscle tension, they saw instructed sighs didn't lead to as much relief as a spontaneous sigh did.
Vlemincx said it might be that being told to sigh was perceived as another task, and so it didn’t function in the same way. Or, it could have been that the person was doing spontaneous sighs already, and adding an instructed sigh didn’t serve as a rest, but pushed them over the line into excessive sighing.
It makes studying sighing tricky. Making people aware of their sighing can change the amount that they sigh, or lead them to place too much attention on their breathing.
Ramirez said that even in rodents, his graduate students are finding there are multiple types of sighs that seem to have different effects on the body and behavior. “It drives them crazy sometimes because the mice’s sighs are so different," he said.
Vlemincx's advice is not to over-complicate sighing. “As long as you’re healthy, don’t force your sighing," she said. "If there’s nothing wrong, don’t fix it. There’s a reason why you’re sighing the way you’re sighing."
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