Why do babies rub their eyes when they're tired?

Parents are taught that when a baby rubs their eyes, it's a telltale sign they're ready for a nap. But why do babies rub their eyes when they're tired? What triggers this behavior, and what purpose does it serve?

"Unfortunately, we can't ask a baby exactly why they're rubbing their eyes," Dr. Rebecca Dudovitz, an associate professor of general pediatrics at the UCLA David Geffen School of Medicine, told Live Science.

"But we do know from human experience that people do tend to rub their eyes when they're tired, and we think it may have to do with just a feeling of discomfort that you get when your eye muscles have been working hard and it's time for a break," Dudovitz said.

The same way your shoulders might need a massage after you sit at a desk all day, the muscles that help your eyes focus feel better after you rub them. Babies spend a lot of their time staring at objects in their environment, and their eyes get tired.

Related: What happens when a baby takes its first breath?

Staring also dries out the eyes — and considering that babies blink only a few times a minute, it shouldn't be surprising that their eyes get dry, too.

"Tears are not just salt water, but mucus near the surface, salt water in the middle, and a layer of oil from the eyelid meibomian glands to prevent evaporation," Dr. Robert W. Arnold, an ophthalmologist at Alaska Children's Eye & Strabismus, told Live Science via email. "Therefore, a healthy tear is a tri-layer. That tri-layer must be renewed and spread over the surface smoothly by blinking."

When we're faced with too much intense visual attention, we don't blink enough. And without enough blinking, that tri-layer can break apart and leave dry patches on the surface of the cornea, the outer, clear part of the eye that covers the iris and pupil. When that happens, eye rubbing may just be a reminder to blink more.

"This is similar to breathing, the rate of which is usually automatic, and sufficient. However, when we are distracted or tired, we may not breathe often, or deep enough," Arnold said. "Therefore, we sigh. Eye rubbing in children may be akin to sighing for eyeball health."

Rubbing your eyes isn't particularly good for you, however. Excessive eye rubbing can lead to vision problems.

One commonly cited reason it feels good to rub your eyes when you're tired is that it reduces blood pressure by stimulating the trigeminal and vagus nerves, which run from the brain to the eyes and from the brain throughout the body, respectively. In some people, this can reduce heart rate by more than 20% in a phenomenon known as the oculocardiac reflex.

But Arnold questions this reasoning. "There is no obvious reason why a kid would feel better with a slow heart rate," he said.

In fact, the oculocardiac reflex can have life-threatening results, including bradycardia — a heart rate below 60 beats per minute — and cardiac arrest.

"We don't think purposeful stimulation of the oculocardiac reflex is a reason for children rubbing the eyes, but it could be a result of excessive rubbing of the eyes," Arnold said.

But most likely, babies rub their eyes for the same reasons adults do: Their eyes are tired and dry, and they're ready for a nap.