[INTRO ♪] [sigh].

Want to do it with me this time? [sigh]. Feels pretty good, doesn't it?

Sighing is one of those universal human experiences. For people around the world, taking a deep breath has a calming power, and it's part of the way we experience and communicate emotion. But it turns out that the assumptions we make when we hear other people sighing … aren't always right.

The reasons we sigh start with biology, because your lungs actually need to sigh just keep functioning properly. When you take a deep breath, you're kind of rebooting them. When you're breathing normally for a while, that's what scientists call eupneic breathing—meaning that it's normal and unlabored.

And if you just let people sit with their eupneic breathing for a while, eventually they'll take a breath that's about twice the usual volume of air, which is what we tend to call a sigh. That's usually followed by what's known as a post-sigh apnea— a little gap between breaths that's a bit longer than normal. This big breath pushes you to fully expand your alveoli— the tiny air sacs that fill up your lungs.

And that helps keep your lungs from collapsing, which is good, because collapsing is a very bad thing that you really don't want your lungs to do. Researchers have found that people's breathing becomes more variable just before they sigh, and it's a little more consistent afterward. So it really is like your lungs get a reset.

Generally, you need to sigh every 5 minutes or so just to keep your lungs in working order. You even sigh in your sleep, although that tends to be less often. But there are lots of factors that can change the pattern of when you sigh— like stress—and that's where the psychology comes in.

Psychologists have known for a while that breathing patterns are related to emotions. But sighing is weird because it seems to be related to both positive and negative emotions. For example, you might sigh because you're stressed or frustrated, but you might also do it because you're feeling relaxed.

One way to explain the contradiction is that sighing could have a lot to do with relief: you do it because you're feeling relieved, or because you're stressed out and your brain is trying to make you feel more relieved. In a 2009 study published in the journal Psychophysiology, a group of Belgian researchers decided to test this idea. In three separate experiments, the team made a total of 115 people listen to an obnoxiously loud white noise turn on and off.

They used different shapes on a screen to signal whether a noise was coming. One shape meant guaranteed irritation, one meant the subject was safe, and one meant there was a 50-50 chance that they'd have to hear the noise. The researchers found that people sighed a little more while they were hearing the noise, but a lot more once it was over.

And, when they were asked about their emotions during the experiment, people who sighed more also rated themselves as feeling more relieved. But it didn't matter if it was relief from the obnoxious sound ending or just from discovering they didn't have to hear the sound. They'd sigh just because of the relief from the anticipation of the stress.

So if sighing has to do with feeling relieved or relaxed, it makes sense that it would be associated with a lot of different emotions. If you're taking a stressful test, for example, you might sigh in relief once you finish a problem, but you might also sigh while you're working on the next problem to try to make yourself feel more relieved and lower your stress levels. Some psychologists have proposed that because people tend to sigh when they're feeling relief from stress, humans learned to interpret sighing from others as a signal of safety.

Which would explain why sighing seems to be a little contagious— it's like your brain is thinking “oh, my friend thinks that tiger's gone? I guess I can take a breath too.” But now that most of us don't spend our days trying to evade tigers, we read into sighing a little differently—and sometimes not as accurately. For example, in a 2008 study, 117 people were given hypothetical stories about someone sighing and asked what they thought the sigh meant.

Usually, they assumed it meant something negative—most often sadness. And if the story was about someone sighing alone, people interpreted the feeling as more intense. But if they were told to imagine themselves in the story, they came up with more varied responses, like maybe they were frustrated or just tired or bored.

So, when you hear someone sigh, it could mean that they're sad— but it's worth keeping in mind that there are lots of other possibilities too. They could be stressed, or frustrated, or even experiencing a moment of calm and [sigh] taking a breath. Or it could just be a totally involuntary response to keep their lungs working properly.

Thanks for watching this episode of SciShow Psych, which was brought to you by our patrons on Patreon. If you'd like to learn more about the weird ways your brain interprets the world around you— and why you shouldn't always trust it—you can go to youtube.com/scishowpsych and subscribe. [OUTRO ♪].