Yawning most often occurs in adults immediately before and after sleep, during tedious activities and as a result of its infectious quality. It is commonly associated with tiredness, stress, overwork, lack of stimulation and boredom, though studies show it may be linked to the cooling of the brain.
In humans, yawning is often triggered by others yawning (e.g., seeing a person yawning, talking to someone on the phone who is yawning) and is a typical example of positive feedback. This "infectious" yawning has also been observed in chimpanzees, dogs, and can occur across species.
Approximately 20 physiological reasons for yawning have been proposed by scholars, but there is little agreement about its main functions.