A yawn is an involuntary reflex where the mouth is opened wide, and the lungs take in a lot of air. The air is then exhaled slowly. During this time, the eardrums stretch, and the eyes may also close tight, causing them to water.
No thought or action has to be taken to produce a yawn, and the process is similar for everyone. Yawning commonly occurs either before or after sleep, which is why it is usually considered a sign of being tired. Yawning also occurs frequently in people who are doing boring or tedious things.
There is also a social aspect to yawning. Yawning appears to be contagious among humans and other animals, and the contagiousness of a yawn is well documented but hardly understood.
Yawning is commonly thought to be a sign of sleepiness or boredom, though this is not always the case.
While someone who yawns may be tired, the heart rate quickly rises during a yawn. This increased heart rate suggests yawning can be a sign of alertness rather than sluggishness.
Yawning, in general, may simply be a way for the body to change the state of awareness it is in:
- Before bed: yawning could be taken as a sign that the body is preparing for sleep.
- When bored: yawning while doing a boring task may be a sign of the brain transitioning from a high level of alertness to a lower one.
- After exercise or sport: yawning after an intense sports activity may be a sign of transitioning from high energy to low energy in the brain.
People may also yawn when changing physical states as well, such as moving from an area of high pressure
to low pressure. This pressure can build up in the eardrums and may cause the person to yawn to release it.