Yawning is one of those behaviors that feels as though it should have a simple explanation. Open mouth, deep breath, stretch the jaw, move on with your day. The problem is that scientists still do not have one theory that explains every yawn.
Yawning is strongly associated with changes in state. We yawn when we are tired, waking up, bored, stressed, or shifting between levels of alertness. It also occurs in many other vertebrate animals.
Does yawning mean your body needs more oxygen?
Probably not. The old explanation was that low oxygen or too much carbon dioxide triggered yawning so the body could take in a bigger breath. Experiments have not supported that as the main explanation.
Yawning certainly changes breathing for a moment, but simply needing more oxygen does not fit the evidence very well.
So if somebody yawns while you are talking, you cannot rescue the conversation by opening a window and declaring the oxygen problem solved.
Yawning often happens when the brain is changing gears
People tend to yawn around transitions in alertness: before sleep, after waking, during boredom, and sometimes before an activity that requires attention.
That has led researchers to think yawning may be involved in regulating arousal or helping the nervous system shift from one state to another. This does not mean a yawn always signals sleepiness. Athletes and performers can yawn before competition or stressful events too.
The common thread may be the transition itself rather than one specific emotion.
Does yawning cool the brain?
Brain cooling is one proposed explanation. A wide jaw movement, changes in blood flow, and the movement of air through the face and upper airway could potentially influence temperature around the head.
Some studies have found patterns consistent with thermoregulation, but this is still an active area of research. It would be too strong to say scientists have proven that the purpose of yawning is to cool the brain.
This is a good example of science being less tidy than a one-line answer. A theory can be plausible and supported by evidence without being the final word.
Why is yawning contagious?
Seeing someone yawn, hearing about yawning, or even reading about yawning can make another person yawn. Contagious yawning appears to involve attention and social-processing systems in the brain.
Humans are not the only animals that show forms of contagious yawning. Researchers have studied it in several social species.
You may also have noticed the practical problem with researching this subject: the experiment can become self-demonstrating very quickly.
Does contagious yawning prove empathy?
You may see the claim that people who yawn after someone else are more empathetic. The relationship is not that simple. Social connection, attention, age, familiarity, and other factors can influence contagious yawning.
It is safer to say that contagious yawning is a social response that researchers use to study perception and behavior. It is not a personality test hiding inside your mouth.
Why do some people yawn more than others?
Sleepiness, sleep schedule, stress, boredom, medications, temperature, and individual differences can all affect yawning. An occasional cluster of yawns is usually ordinary.
Very frequent or unusual yawning can sometimes accompany medical conditions or medication effects, especially if it appears suddenly with other symptoms. In those cases, the yawning pattern matters more than a single yawn.
The best short answer is that yawning seems connected with regulating body and brain state, and in humans it also has a contagious social side. We know quite a lot about when yawning happens. We are still working on the final why.
The same body systems connect with Why do we sneeze?, another everyday effect with its own underlying mechanism.
And that’s the short explanation.
Related explanations: Why do we sneeze?, Why do we get goosebumps?, and Why do we blush?