It takes about six seconds. Slowly, you take in a deep breath, exhale more quickly, stretch out your arms, contort your face, and your narrowed eyes may tear a bit. You have just yawned, something that almost all vertebrate animals do many times a day — by some estimates, 10 times per hour, though more often in the early morning and late evening. Does it mean you are tired? Bored? Trying to give someone a hint?
As common as it is, little is known for sure about yawning, but it is probably a myth that yawning always indicates a need for sleep. It is true that people often yawn as they are ready to retire for the night; but it is also true that it happens when first arising in the morning, and at other times during the day depending on a variety of factors such as arousal level, distraction and even seeing someone else yawning.
Because breathing takes in oxygen and removes carbon dioxide, theories in the past about why we yawn centered on the assumption that it was a reflex in response to low oxygen or high carbon-dioxide levels. This theory lost favor after a study in 1987 in which volunteers subjected to high oxygen levels did not yawn less, and after high carbon dioxide exposure did not yawn more.
Any explanation about why we yawn will have to explain several interesting observations:
There seems to be no shortage of potential explanations for yawning. Some of the most reasonable theories follow, though none has been proven:
It is probably the power of suggestion that leads one person to yawn after seeing someone else yawn. If the theory about nonverbal group communication is true, as it seems to be for baboons, the contagiousness of yawning may be an involuntary, genetically programmed phenomenon; once one person in the "tribe" yawns, others do so because this behavior pattern helped our evolutionary ancestors to communicate with one another.
Believe it or not, yawning may be a sign of disease. Although rarely the first sign, excessive yawning has been observed among people with multiple sclerosis, amyotrophic lateral sclerosis (ALS, also called Lou Gehrig's disease), after radiation injury (as during radiation therapy), and among people treated for Parkinson's disease. Rarely, it may even signal the onset of migraine headache. On the other hand, yawning seems to occur less frequently among people with schizophrenia. The reasons for yawning more or less often in certain diseases are unknown.
Researchers have noticed in animals and humans that electrical stimulation of certain parts of the brain can provoke yawning. In addition, certain chemical messengers in the brain (including dopamine and ACTH) seem to be important in yawning as well. Yawning seems simple enough, but is actually complex behavior serving an uncertain purpose.
Don't take it personally if someone yawns in your face. While it is possible they are bored, tired or trying to send you a message, there's plenty about yawning we do not yet understand. Maybe they just want to be left alone, need to stretch out the small airways of their lungs, or are preparing to focus even more closely on what you are saying. Then again, maybe you should take a hint and put away those slides of your summer vacation.
Robert H. Shmerling, M.D. is associate physician at Beth Israel Deaconess Medical Center and associate professor at Harvard Medical School. He has been a practicing rheumatologist for over 20 years at Beth Israel Deaconess Medical Center. He is an active teacher in the Internal Medicine Residency Program, serving as the Robinson Firm Chief. He is also a teacher in the Rheumatology Fellowship Program.