It’s actually pretty normal and you probably do it without realizing it. Occasionally the lungs just need to absorb a little extra oxygen to catch up. You ever watch a dog sleep and every now and then they just take a big inhale? Same thing.

Found this neat source:

“A sigh is a long, deep breath that is often viewed as an expression of stress, sadness, exhaustion or relief. However, the most frequent sighs are unnoticed and occur spontaneously every several minutes, about a dozen times per hour.”

. . .

“The lung is composed of hundreds of millions of alveoli, the gas exchange units at terminal ends of the respiratory tract, each of which is about 200 micrometers in diameter. During normal breathing, alveoli spontaneously collapse, a pathological condition known as atelectasis. A sigh is hypothesized to reverse any alveolar collapse, because it is a large breath that re-expands all alveoli, filling them all with air.”