Physiological sighs are spontaneously engaged to correct imbalances in carbon dioxide and oxygen levels.

“And it's observed in animals. You might see this in animals that are tired. When animals or humans get tired, they tend to start underbreathing a little bit, and that can often disrupt the balance of carbon dioxide and oxygen. And right before a dog will go down for a nap, for instance, you'll notice that it'll do this double inhale, exhale. people when they are sleeping, if they hold their breath for a period of time, which, frankly, all of us do periodically throughout sleep, they will engage a spontaneous physiological sigh.”

“But the physiological sigh was discovered in the 1930s as a spontaneous pattern of breathing that occurs involuntarily in sleep in order to reinflate the lungs and offload carbon dioxide and bring oxygen back into the system on the subsequent breath.”