A yawn is a deep breath in with the mouth wide open, often with the eyes closing and a stretch of the body, and yawns are catching: when you see someone yawn, you often yawn too. Yawning can make a person yawn even if they only hear or think about a yawn. Babies yawn before they are born, and fetal yawns were recorded with 4D ultrasound between 11 and 20 weeks of pregnancy, so yawning appears long before birth. Newborns and infants yawn often, which may be linked to their frequent switches between sleeping and waking. Contagious yawning does not fully show up in children until about age four or five.

Other animals yawn too, including cats, dogs, birds and fish, and dogs can catch a yawn from a person, too. Chimpanzees, our closest living relatives, yawn on their own and also catch yawns from each other. We are much more likely to catch a yawn from a close loved one than from a stranger. Researchers think contagious yawning is related to empathy, our ability to understand and share the feelings of another, though the idea is still being tested. The communication theory says a yawn may tell the people around you that you are bored or stressed, or that the place is not right, which could be why yawns spread. People also tend to yawn when they are nervous or before an important event.

In a 2007 study led by psychology professor Andrew Gallup, people who were warmed yawned more at pictures of yawning, and people in a cooler place or with cold packs on their foreheads yawned less. In 2014, researchers from the University of Vienna measured contagious yawning outdoors in Vienna in summer and winter and compared it with an earlier Arizona study: people in Vienna yawned more in summer, and people in Arizona yawned more in winter. The Vienna team found that contagious yawning was limited to an optimal range of temperatures around 20 degrees C (68 F). It faded when it was about 37 C (99 F) in Arizona summer or near freezing in Vienna winter. The authors suggest yawning helps regulate brain temperature. In birds such as budgerigars and parakeets, yawning-like behaviors are thought to help regulate brain temperature.

A yawn lasts about four to seven seconds, and adults yawn about eight to nine times a day on average. Yawning is started in the brain by the hypothalamus and brainstem, with chemical messengers called neurotransmitters. During a yawn the throat widens to three or four times its normal size, and the ears may pop as the eustachian tubes open. Muscles in the ear called the tensor tympani are activated, which may reset the range of movement and sensitivity of the eardrum and so help us monitor the world after we may have tuned out. A Swinburne University article says we do not have evidence that points to the exact purpose of yawning, even though many triggers and the mechanism are well known.

Nerves are at work in a sneeze and in a hiccup as well. In a sneeze, nerves in the nose send a message to the brainstem, and many muscles then work together to push the air out. A sneeze has three stages: a big breath in as the diaphragm and rib muscles contract, a squeeze as the soft palate seals the back of the nose, then an explosive breath out as the glottis opens. Your diaphragm is a muscle under your lungs that helps you breathe. Hiccups are a reflex: the diaphragm contracts suddenly, which causes a sudden intake of breath that is cut off when the glottis (the gap between the vocal cords) closes. The vagus and phrenic nerves supply the diaphragm muscle, and irritation of these nerves is one cause of long-lasting hiccups.

Words to know

• contagious: spreading to others
• empathy: sharing feelings
• ultrasound: body scan
• neurotransmitters: brain messengers
• glottis: throat gap