I tend to yawn when someone around me does.
I found this also at ScienceDaily:
University of Albany researchers suggest that yawning does not promote sleep but helps mitigate the need to sleep. Since yawning occurs when brain temperature rises, sending cool blood to the brain serves to maintain optimal levels of mental efficiency. Therefore, the psychologists say, when mental processing slows and someone yawns, the tendency for other people to yawn contagiously might have evolved to promote group vigilance as a means of detecting danger.