When you are mentally or physically tired, carbon dioxide causes deep breathing in the throat, and the body expels the carbon dioxide, George Bubenik said. But a yawn also can be a signal between two people, a way of communicating. You take that cue from me, and you yawn too. People are a lot like animals, and yawning is one way animal herds transmit moods. A yawn that results from merely thinking about yawning is a subconscious response based on memory.
Humans have a great capacity for memory.
If you are flying off to somewhere like Europe or Australia, count on it taking a few days for your mind and body to get in sync. Flying through different time zones disrupts the body's perception of time. Bubenik is conducting research on Melatonin, a natural substance. One of its benefits is preventing jet lag. Melatonin also regulates digestion and diurnal rhythms.