It’s a little embarrassing.
Despite being about as familiar and as commonplace as you can get, we still don’t have a clear understanding of why humans yawn.
We know we start yawning early. We know we yawn when we’re tired. We know we yawn when we’re bored. And we know that yawning can sometimes be contagious. But the function, the why, has been elusive.
A new paper by Giganti and Zilli ties together a couple of yawning’s features: that the amount people yawn varies throughout the day, and that yawns can be contagious. But does the contagiousness of yawns vary throughout the day?
First, their got subjects to record their sleepiness and yawns over several days in a logbook. Now, self-report data always has concerns about accuracy (do people remember right, record immediately, or write things to make themselves look good), but because this is just baseline data on a fairly neutral subject, we’ll move on.
Second, they tested their subjects several times on a single day, showing them either people yawning or smiling, and recording the yawns generated in response.
Most of the results are not surprising. More yawns when people report their feeling sleeping. More yawns when people are shown pictures of yawns compared to smiles. The main finding of interest is that they claim there is a significant interaction between the time of day and how contagious yawns are.
Yawns are most contagious at 7:30 pm.
At every other time they tested during the day, the spontaneous yawns (determined from the logbook) were greater than the yawns triggered by the experimental stimulus of watching yawning people. Only in the early evening did the triggered yawns exceed the expected rate. They didn’t perform any post-hoc tests on their groups, however, so it’s not clear if any of the other four times had significantly more spontaneous yawning than induced yawning.
Interaction effects have a reputation as being unstable, so it would be very interesting to see these results replicated.
These results might nudge us a little closer to understanding why we yawn. The paper suggests there are at least two kinds of yawns. A spontaneous yawn and a yawn in a social setting may well be entirely different beasts that need different explanations. Maybe the reason yawns have thwarted our efforts to understand them is that a single explanation for yawns you make alone completely fail when you try to apply it to yawns you make around other people.
P.S.—Yawns during the writing of this post: At least three. I hope that if you yawn during this post, it’s a contagious yawn, and not one triggered by boredom!
Giganti F., & Zilli I. (2010). The daily time course of contagious and spontaneous yawning among humans Journal of Ethology. DOI: 10.1007/s10164-010-0242-0
Tiger photo by by Tambako the Jaguar on Flickr; baby photo by Twob on Flickr. Both used under a Creative Commons license.