Good morning on this, yawn, Monday.

Up and at ‘em — and back to work again.

As we go about our morning — standing in the coffee queue, waiting for our train, or trying to make it through an early meeting — does it seem unusual that almost out of nowhere, we may squint our eyes, drop our jaws and yawn?

What’s up with that?

A couple of New York City doctors told us that nobody quite knows.

Yawning is an ancient evolutionary behavior, but it’s unclear why we do it, according to Dr. Orrin Devinsky, a professor in the departments of neurology, neurosurgery and psychiatry at NYU Langone Medical Center.

“Whatever it is, its origins go back long ago to when our ancestors were literally living in the oceans and breathing through our gills,” he told us. “We know the circumstances that yawning is associated with, but what we don’t know is why this reflex first evolved in fish and later in reptiles, and why it has been maintained over hundreds of millions of years,” Dr. Devinsky said.

A reflex? Yawning is hard-wired into our nervous system, like sneezing as a way of cleaning out our nasal passages, or coughing as a way of expelling mucus in our throat, but “yawning is probably the least well-understood of all of these,” he said.