What is the point behind this deceptively simple act? Why do our bodies make us stretch and yawn even before we're properly awake?
I find that stretching after a long period of inactivity (especially after sleeping) is almost an instinctual reaction for me. It feels awesome, and it leaves me refreshed. But how can a simple stretch do that? And why does my body compel me to do it without even asking for my opinion? What does my body get out of it?
Table of contents
Some light pandiculation
The act of involuntary stretching while yawning is referred to as pandiculation in humans. The behavior, however, is far from unique to us. It's been observed in many different species, particularly during transitions from periods of low to high activity.
"Almost all vertebrates yawn," Olivier Walusinski wrote in 2006, "testifying the phylogenetic old origins of this behavior". He further adds that such behavior can be observed in infants as young as 12 weeks, and "remains relatively unchanged throughout life".
Another paper published in the journal Neuroscience & Biobehavioral Reviews discusses the possible origins of mammalian sleeping patterns in the wakefulness, sun basking, and sleeping states of our reptilian ancestors. It's a very interesting read. In the context of what we're discussing today, however, one point stands out: the paper suggests that stretching and yawning stem from post-basking risk-assessment behaviors.
"Post-basking behaviour always begins with a passive exploration of the immediate environment," for things such as threats, food, or mates, the authors write. "This behaviour [...] consists of the suspension of current behaviour, to be replaced by head dipping movements, eye scanning, rearing and adopting stretch attending postures."
So not only are yawning and stretching widely seen in the animal world, they're also probably very, very ancient behaviors. We pick them up early and stick with them for life. So surely they have a purpose -- but what?
While we don't know for sure, we have some pretty solid hypotheses.
Stretching as a 'hardware reset'
Promote me to Captain Obvious here, but sleeping is a very passive ability. Our bodies are made to move, however, and such a long period of inactivity leaves them drowsy in a sense. Stretching is our brain's way of checking if all muscles are still working properly while giving them a nudge that it's time to get to work.
Beyond revving up muscles, stretching offers a gentle transition from sleep to wakefulness for the rest of the body as well. We're basically bags of meat filled with fluid. During the day, fluid tends to accumulate in the legs. At night, which we usually spend lying down -- for example in the supine position -- gravity instead pulls these fluids towards the spine, torso, neck, or head, wrote Laura White and Douglas Bradley in the Journal of Psychology back in 2013. Stretching helps to gently push these fluids back into their usual place. It's probable that this measure is designed to prevent fluid build-ups from injuring muscles during more strenuous activity.
It also helps work out any stiffness or tightness in your muscles and joints caused by spending an extended amount of time in a single position. In the long term, this helps maintain a wide range of mobility even if we don't engage in such activities. In the short term, pandiculation might be a quick way to carry our bodies out of REM sleeping patterns (when motor activity is inhibited) and into a state of readiness, so we can react to any danger -- just like reptiles after basking,
Stretching as a 'software reset'
Pandiculation also helps improve blood flow and reduce stress by jump-starting the parasympathetic nervous system (PNS) -- the branch of the nervous system that handles involuntary activity such as controlling heart rate, endocrine functions, or digestion. Stretching jump-starts the PNS, which, in turn, revs up all those background processes that keep you awake and alert throughout the day. The modicum of movement also increases your heart rate -- which is slowest just before rising -- pushing blood to the muscles in the extremities.
Stretching also offers the brain a chance to recalibrate its communication with muscles. As you stretch, the brain sends progressively stronger signals to your muscles. These contract in response to the signal, and stop as the filaments of actin and myosin they're made of come close to their breaking point. This feedback lets the brain calibrate how strong a signal it needs to send to the muscles for various tasks, and lets it know how much strain the muscle can safely take.
Finally, stretching simply feels good. It's a form of progressive relaxation that helps reduce feelings of stress. Stretching feels good because it's one of those things that satisfy our homeostatic drive: along with eating, having sex, and satisfying other bodily functions, stretching helps us stay healthy -- so our brains reward it by making us feel nice.
Ok, but why do we stretch?
Animal models suggest that pandiculation is regulated by the same networks in the limbic system (the 'lizard brain') that handle basic survival instincts. Some patients paralyzed on one side of their body due to motor cortex damage will still raise both arms when they yawn -- which suggests it is a function of the limbic system, rather than the motor cortex, which is prompting this behavior.
This would also explain why we naturally stretch when we wake up, despite not consciously trying to.
"Of 40 stroke patients attending a rehabilitation department, 32 (80%) had associated reactions affecting the hemiplegic arm," reports a study published in 1982 in the Scandinavian Journal of Rehabilitation Medicine. "These involuntary movements nearly always occurred in association with yawning and less frequently with stretching, coughing, sneezing and laughing."
The other part of pandiculation, yawning, is likely more brain-centered. Where stretching helps bring your muscles online, yawning likely does the same for our pounds of gray matter. It helps cool the brain and likely makes it more alert -- we yawn when we're sleepy in an effort to stay awake, and yawn when we're bored in an effort to keep us on the task at hand.
That's not to say that scientists know for sure why we yawn. The only evidence-support function of yawning has been to help thermoregulate the brain. Although it’s relatively small, the human brain uses approximately 40% of our metabolic energy, which means it’s also more likely to overheat. When we yawn, we ingest a gulp of air that goes into contact with our nasal and oral cavities, which are directly linked to the brain through countless blood vessels. Furthermore, when we stretch our jaws, we increase the blood flow to the brain, which helps the relatively colder air slightly reduce the temperature of our brains.
Here to stay
You probably enjoy pandiculating -- I can't blame you. But, even if you didn't, chances are it's here to stay. Not only is this behavior likely very useful for our bodies, but it's also engrained so deep in the animal brain that almost all vertebrates do it.
So stretch your arms high, yawn with gusto, and enjoy that little shot of dopamine that comes with pandiculation.