The other evening during the 6pm class, Anna noticed that she would yawn prior to each set of Press.  When she brought it up to everyone it sparked quite a bit of discussion about the possible causes and the general theory behind why humans, and just about all animals, yawn.  Ideas were tossed around like the body needs more oxygen and the big breath associated with the yawn raises the body’s oxygen levels, or the idea that a person yawns as they transition between different states of alertness (awake to tired, interested to bored, or even blah to amped up for a heavy lift).  This piqued my curiosity and I decided to do a little research on yawning to add to the discussion.

Over the years yawning has puzzled the scientific community as many theories have been introduced, tested, and dismissed.  One of these theories is that yawning increases the oxygen level in the body.  This theory seemed plausible, and in some cases may indirectly lead to yawning, but studies have found that different parts of the brain control breathing and yawning, the body’s oxygen level actually decreases a bit from a yawn, and human embryos have been observed yawning in the uterus where no breathable oxygen is present.  Another theory is that we just yawn when were bored or tired.  This may be the result of low oxygen levels in the paraventricular nucleus (PVN) of the hypothalamus of the brain, the “yawning center” of the brain.  The PVN contains a number of neurotransmitters and chemicals that can induce yawning.  Once stimulated, the cells of the PVN activate cells of the brain stem and/or hippocampus, causing yawning to occur.  One in particular, adrenocorticotrophic hormone (ACTH), surges at night and prior to awakening, and induces yawning and stretching behavior in humans [1].  I’ll readdress the yawning when we feel tired later in the post.

YAWWWWNNNN…super interesting stuff so far, right?  If you’re like me, you’ve already yawned a half-dozen times just reading this post.  Current studies suggest that the evolutionary reason for yawning has to do with keeping our brains cool.  Yes…you read that correctly.  One of the leading researchers is Andrew Gallup [2] who has published many studies on the issue.  He’s found that, “Brains are like computers.  They operate most efficiently when cool, and physical adaptations have evolved to allow maximum cooling of the brain” [3].  Two studies in particular are of note.  The first study was done on humans in 2007 and investigated the contagious nature of yawning and ways to mitigate it.  Gallup and his colleague found people yawned almost 30% more when warm or room temperature packs were held to their foreheads than cold packs [4].  In a follow-up study published in 2009, Gallup and his team studied the relationship between yawning and ambient temperature on budgerigars, Australian birds who do not exhibit the contagious yawning behavior like most other animals.  He found that the birds were prone to yawning as the temperature grew warmer supporting his hypothesis that yawning serves a thermoregulatory function [5].  Yawning serves to reinstate an optimal brain temperature by acting like a radiator.  If air in the atmosphere is cooler than brain and body temperatures, taking it in quickly cools facial blood that, in turn, cools the brain and may even alter blood flow [3].  In my experience, I’ve caught myself yawning in situations just like Anna; prior to a workset, during or after warm-ups, before a game or race, etc.  It’s interesting that in all these situations I don’t recall more than having broken a slight sweat.  I say this because sweating serves more than just cooling the brain, it helps maintain our core body temperature within safe levels.  I speculate that at a certain point yawning ceases to be enough of a cooling mechanism for our brains as our core temperature rises and the sweating takes over.  Perhaps we’ll discuss in a future post, but back to yawning for now.

Both exhaustion and sleep deprivation have been shown to increase deep brain temperatures, which may explain why we often find ourselves yawning when we feel tired [3].  This often leads us to link yawning as a precursor to sleep, but the yawn is actually our brain’s reflex to jump start itself back to a more heightened state of alertness.  I’ll touch on the contagious nature of yawning in a future post, but the next time you find yourself yawning know that your brain is trying to cool off and stay alert.

References:
[1] http://www.scientificamerican.com/article.cfm?id=why-do-we-yawn-when-we-ar
[2] http://evolution.binghamton.edu/evos/people/acgallup/
[3] http://dsc.discovery.com/news/2008/12/15/yawn-brain-head.html
[4] http://www.epjournal.net/filestore/ep0592101.pdf
[5] http://www.sciencedirect.com/science?_ob=ArticleURL&_udi=B6W9W-4TVJJTG-3&_user=7738676&_coverDate=01%2F31%2F2009&_rdoc=1&_fmt=high&_orig=search&_sort=d&_docanchor=&view=c&_acct=C000072704&_version=1&_urlVersion=0&_userid=7738676&md5=5bee9978706f852bda9d3bede90762d3  

Further Reading:
http://www.ncbi.nlm.nih.gov/pubmed/19657685
http://scienceblogs.com/grrlscientist/2008/12/yawning_and_thermoregulation_i.php
http://content.karger.com/ProdukteDB/produkte.asp?Aktion=showproducts&searchWhat=books&ProduktNr=253772

REST DAY 06.06.10