My father used to say ‘I am not tired, I just need oxygen.’ Was he right? Why do we yawn? What are your theories about yawning?
My personal interest in the phenomenon of yawning goes back to years in vocal rehabilitation as a voice therapist. Almost all vocal hygiene techniques suggest the benefits of pretend-yawning during vocalization. The goal is to enlarge the vocal tract in order to create room for the voice to resonate. The practice also serves to coordinate the whole system, from palate to vocal cords to diaphragm, to engage with increased muscular tone, because you cannot sing or speak on a professional level for an extended time if your vocal muscles are ‘relaxed’.
In my experience the practice of yawning for vocalization leads to several common side-effects: the contagiousness of yawning becomes very evident, and then there is the phenomenon of something I call ‘yawn addiction’. In my own voice training I often could not stop yawning – for what seemed like days at a time. Based on this experience and similar experiences with students, I developed my own theories – about the organism’s need for oxygen (my father’s hypothesis), and the need for muscular expansion and toning of the vocal tract and diaphragm.
Then I recently had a student, a nurse practitioner, telling me that yawning is good for us, because it produces surfactant! Surfactant? Never heard of it! I got very excited and learned that surfactant in the lungs is produced by the alveoli; its purpose is to reduce surface tension of pulmonary fluids in order to promote elasticity of lung tissues.
With such factors in mind, I set out on a literature survey of yawning. To summarize: yawning is both, a reflex and a learned behavior!
Arousal/wakefulness: Yawning can be observed in ultrasound images of embryos after the seventh month of pregnancy. (There goes my father’s oxygen theory – embryos don’t get their oxygen from inhaling the amniotic fluid surrounding them!)
Infants are only observed yawning when they awake from sleep/naps.
In humans (school aged and older) fatigue and boredom are the most common stimuli to yawn. Studies suggest that yawning leads to higher brain activation and arousal levels, measured by a rise of heart rate and skin conductance (both are sympathetic nervous system responses, see my blog on Orienting). One study found the arousal increase through yawning being equivalent to a single dose of caffeine! Is that why we yawn when we wake up?
Contagion/ involuntary transmission/ being affected: Contagious yawning develops during toddler and preschool years and can only be reliably induced after six years of age.
Once learned, it seems to depend on the degree of familiarity, following along the line of family, friends, acquaintances, strangers. It has also been interpreted as a measure of empathy.
Contagious yawning can cross species; dogs above the age of about seven months, are observed to catch yawns from their significant human.
In mammals and other animals contagious yawning is understood as a way to regulate group behavior, like coordination of sleep or raising activation levels.
Brain Cooling (mammalian brains operate best within a narrow range in temperature): Studies of animals and humans provide growing evidence that yawning occurs before, during and after instances of unusual heat regulation or heat stress. The contraction and relaxation of facial muscles during yawning, increases blood flow in the face, which helps to dissipate heat through veins. The deep inhalation through a gaping mouth brings colder air into the lungs, tearing eyes may likewise play a role in heat dissipation from the skull.
Neurological disorders such as MS, epilepsy, migraine, stress, anxiety, head trauma and stroke lead to increased body core temperature and abnormal heat regulation; patients with such conditions can experience excessive yawning, which can lead temporarily to a lessening of their symptoms.
Back again to my dad’s theory of oxygen replenishment: It is an old and still very commonly held belief. However, studies have shown that yawning frequency is not affected by either an increase of CO2 or by providing subjects with pure oxygen. Some studies indicate that oxygen intake might even be lower during yawning than with regular breathing.
And for my student’s comment that yawning produces surfactant, the wikipedia link informs us: pulmonary stretch receptors … are mechanical receptors found in the lungs… Increased firing from the stretch receptors increases production of pulmonary surfactant.
I yawned this morning after an hour of vigorous exercise! Was I sleepy? No. Did someone else yawn? No. Was my body and brain temperature elevated? Possibly…
Again, why do we yawn? It seems that the following contribution from a pioneer in yawning research still holds true: “Yawning may have the dubious distinction of being the least understood, common human behavior.” (Robert Provine, 1986)