Researchers are starting to unravel the mystery surrounding the yawn, one of the most common and often embarrassing behaviors. Yawning, they have discovered, is much more complicated than previously thought. Although all yawns look the same, they appear to have many different causes and to serve a variety of functions.

Yawning is believed to be a means to keep our brains alert in times of stress. Contagious yawning appears to have evolved in many animal species as a way to protect family and friends, by keeping everyone in the group vigilant. Changes in brain chemistry trigger yawns, which typically last about six seconds and often occur in clusters.

“What this tells us is it’s a very complicated system, and there are probably many different roles for yawning,” says Gregory Collins, a researcher at the University of Texas Health Science Center in San Antonio who has identified some of the chemical processes at work in the brain.