The second inhale during cyclic sighing helps reinflate collapsed alveoli in the lungs.

“Those double inhales are kind of important because what they do is they maximally fill all those little sacs in your lungs, and then when you breathe out, you're exhaling as much of the so-called carbon dioxide in your system as possible.”

“So, there are these mechanisms for these physiological sighs. So, just like with the collapse of the lungs, where you need a big pressure to pop it open, it's the same thing with the alveola.”

“It turns out that we're sighing all the time. And when I would ask people who are not particularly knowledgeable that have read my papers or James Nestor's book or listened to your podcast, they're usually off by two orders of magnitude about how frequently we sigh on the low side. In other words, they say once an hour, 10 times a day. We sigh about every five minutes, and I would encourage anyone who finds that to be a unbelievable fact, is to lie down in a quiet room and just breathe normally, just relax, just let go, and just pay attention to your breathing and you'll find that every couple of minutes, you're [inhales] taking a deep breath and you can't stop it. It just happens.”

“Now, a normal breath is not enough to pop them open, but if you take [inhales] a deep breath it pops them open. - Through nose or your mouth? - Doesn't matter. - Okay. - Doesn't matter. - Or- - It just increased that lung volume 'cause you're just pulling on the lungs, they'll pop open every about every five minutes. And so, we're doing it every five minutes in order to maintain the health of our”

“Now, every five minutes or so, whether or not we are asleep or awake, we do what's called a physiological sigh, which is a big, deep inhale, often a double inhale followed by a long exhale.”

“In fact, that's what happens. We know that during that second inhale, even if it's just a very sharp, short inhale, the extra physical vigor that's required to generate that second inhale causes those alveoli of the lungs, which may have collapsed-- and, indeed, in between breaths and often even just through the course of the day and especially if we get stressed, those alveoli of the lungs start to collapse.”

“In fact, physiological sighs were not discovered by me at all. They were discovered by physiologists in the 1930s, who found that when people underbreathe, they have a buildup of carbon dioxide in their system. And even though carbon dioxide is essential for life, you don't want too much of it in your system. And that people, whether or not they were asleep or awake, would engage a physiological sigh spontaneously, subconsciously.”

“the double inhale reinflates those little sacks of the lungs and then when you do the long exhale that long exhale is now much more effective at ridding your body and bloodstream of carbon dioxide which relaxes you very quickly when you're feeling stressed”