|MadSci Network: General Biology
This is a very interesting question, but I'm afraid that the answer as to why we yawn is still quite a mystery to science. Actually, there are several aspects of human/vertebrate behavior that remain quite mysterious - such as yawning, shedding tears while crying, sleeping, etc. Scientists have simply failed to provide satisfying reasons for these behaviors to exist - though there are of course many theories - all unproven.
So let us consider yawning. When do we yawn? Generally when it is very late and we are tired, but also in the morning when we get up. We also have other associated behaviors - such as stretching, having "heavy" eyelids, etc. So why do we do these things? I will offer several popular theories and then present evidence that suggests that each of these ideas is probably false. Then I will present my own theory and propose several simple experiments to test these ideas.
The standard theories.
1. Yawning is induced by hypoxia (low oxygen levels in the blood stream).
The idea behind this (the most popular) theory is that yawning causes you to take a deep breath when you need some extra oxygen - perhaps because you are tired are have not been breathing enough. These are MANY problems with this idea, but the main problem is that yawning occurs only when you are tired and does not occur in the many other instances of hypoxia. For example, if you exercise hard you need more oxygen - but your body makes you breath harder, not yawn! If you are brought from, say, sea-level to an elevation of 20,000 feet within a time period of 5 min, you will immediately faint due to severe hypoxia, but you will not yawn.
2. Yawning is a method for communicating to other individuals your physiological state of being tired.
This idea is that several human behaviors are for communication purposes. Thus, smiling is to let people know that you're happy, crying is to tell people that you are very sad (and possibly need help), the facial expressions of anger and to let people know that you are mad, and yawning is simply to let people know that you're tired. The problem with this idea is that nearly all vertebrates yawn, including fish, alligators, turtles, etc. Yet it is hard to believe that your goldfish is trying to communicate its feelings or tiredness! These lower vertebrates don't show any other "feeling" communication behaviors (such as smiling, purring, crying, etc.) Thus it does not seem likely that they yawn as a communication device. This does not mean that humans don't use yawning to communicate feelings, but yawning certainly didn't originate for this reason.
3. Yawning is used to stretch out the lungs to prepare them for use.
Perhaps the lungs need a little stretching every once in a while to get out the wrinkles, etc. This would explain why yawning is often associated with stretching other muscles (like the arms, etc.) So all you are doing is preparing the body for a hard day of work. However, this would not explain why you yawn when you get tired before bed - because sleeping definitely does not qualify as a "hard day of work." You don't need to prepare you lungs for sleep since sleep requires much less oxygen than being awake.
Now I will do my best to provide a possible rational explanation/theory for human yawning. That me first suggest that there are actually two different types of yawning - let's call them type I and type II. These are defined as yawning associated with skeletal muscle stretching (type II), or yawning by itself (type I). My proposition is that type I yawning ONLY occurs at night before you go to bed, and that type II yawning ONLY occurs in the morning as you are waking up. Thus, type II yawning is explained by theory #3 - that you are stretching your lungs to prepare them for a day of breathing. Type I yawning is explained by theory #2 - that you are communicating the fact that you're tired to other people. So my theory is actually a combination of theories #2 and #3.
My idea would be that Type II yawning is the only form of yawning used by lower vertebrates and even most mammals, like cats for example. Cats very often yawn and stretch when waking up in the morning, but you will rarely see a cat yawn when it is tired in the evening - so they only have type II yawning. The idea would be that we evolved from a mammal that, like the cat, only had type II yawning. However, for very social animals like our ancestors, evolution selected for a way to communicate being tired. Basically, yawning was adopted for this purpose - and hence type I yawning evolved. This might have been useful for many purposes, such as synchronizing the sleeping schedules of a group of social animals that lived together. This would be the reason yawning seems contagious. When you see someone yawn, your brain makes you become more tired yourself, and so you start yawning as well. This helps everyone go to sleep at about the same time.
So - in science when you propose a theory or idea, you should also propose experiments that might be done to test your ideas. For example, my idea proposes that in the evening (when type I occurs) yawning is for communication or display purposes only - i.e. there is no reason to take a huge deep breath to stretch the lungs out. Thus, a prediction might be that someone would take much larger breaths while yawning in the morning than in the evening - and this could easily be tested. It should also be tested that animals like cats and alligators definitely don't have type I yawning since I'm only going by my own anecdotal observations - this needs to be tested more carefully in a controlled study. Another prediction is that only type I (but not type II) yawning is contagious - also easily testable. I'm sure you could think of lots of other tests to either support or refute this theory. In fact, I REALLY encourage you to try to carry out some of these experiments yourself!!! I'm not aware of anyone doing these very simple experiments - so you would be making new discoveries!
Please feel free to contact me by E-mail is you want to discuss this more.
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