Why do we yawn? and why is it contagious? / why does our nose run when we have a cold/cry?

Yawning is most common when we're tired or bored, and occasionally randomly too! But why?


To be brutally honest, it's not crystal clear why we yawn, but there are a few very convincing and interesting theories!

Because you're not breathing properly

• When you're bored, your heart-rate will be lower than normal because you aren't really doing much, your brain realises this is the case and makes you yawn so you get more oxygen in and get your processes back up to scratch.
• However some studies have shown breathing in more oxygen has little effect on decreasing yawning

Keep the lungs lubricated

• Your lungs need to be kept lubricated with what's called surfactant, an oily substance used to keep the lungs open, which kinda makes sense, otherwise it would be harder to breathe after every breath (lungs must be moist so oxygen transfer can happen between alveoli).
• Every time you yawn, surfactant is redistributed.

Another theory suggests it's not about tiredness, but more the CO2 output

• It's well known that carbon dioxide is the waste product, so your body needs to expel it. However there's not many opportunities to do that when you aren't breathing a lot. So this theory suggests that you yawn to increase the about of CO2 you breathe out, keeping your levels safe.

I've read a ton of theories on this, and the latest seems the most sensible, after all your body is pretty picky about the conditions it's in.

Why is it contagious?


It's not entirely known why, however studies have shown that similar emotional events bring communities closer, and yawning may be one of them, yawning with a fellow human being gives you a sense of a connection.

Another theory is the root of empathy, we consider others and when they yawn, we do the same to show our feelings.

Why do you cry a bit when you yawn?

You may have noticed sometimes you tear up when you yawn. This is due to the lachrymal ducts which holds your tears, when you yawn, some muscles in your face contract which squeezes this sac, and it produces tears from the tear duct.

On the side: why does your nose run when you have a cold or when you cry?

• When you have a cold, the sinuses are covered in ependymal cells. This produce loads and loads of protein rich fluid to wash out all the bacteria.
• Normally, when the lachrymal ducts which hold your tears let them drain down the back of your nose and down your throat (you don't feel it). But when you cry, you make significantly more tears and instead of draining down the back, they just flow down your nose!