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Summer is winding down here in the Northern Hemisphere, which means we’ve started to reflect on our long days at the lake and all the times we’ve jumped into swimming pools. And there’s one thing we’ve noticed:.

Being immersed in water is really relaxing. As it turns out, though, this isn’t just because of the warm weather or the sound of the waves. There’s something larger at play here, called the mammalian dive reflex.

And you can actually take advantage of it all year round, no pool required. When you dunk yourself in water, this reflex kicks in, and your body does some pretty interesting things. Mainly, as soon as your face makes contact with the water, it triggers a reduction in your heart rate — called bradycardia.

The effect happens very quickly, and studies have shown that the mammalian dive reflex can drop someone’s heart rate by ten to 25%! This is what results in that nice, peaceful feeling. Multiple studies have shown that when your heart rate slows down, you feel calmer.

This is also why breathing exercises are so relaxing — although in that case, you’re slowing your heart rate through careful, slow breaths. But while it’s a nice bonus, this reflex doesn’t exist to help you chill out. Instead, research suggests it’s an evolutionary adaptation that allows us to stay underwater for an extended period of time.

After all, lowering your heart rate ultimately reduces the amount of oxygen entering your bloodstream, and it means your body can immediately start conserving oxygen. Which makes sense because, you know, we kind of stop breathing when we’re underwater. Additionally, the mammalian dive reflex causes your body to divert blood from your extremities.

Instead of getting pumped out to your fingers and toes, the blood is redirected into your lungs and other vital organs. This helps preserve your body’s temperature in cold water, and might even help protect you from the powerful crush of water pressure at deep depths. This reflex exists in all kinds of mammals, from aquatic ones to terrestrial ones — including rodents and primates.

Even newborn babies exhibit it! And what’s really interesting about this reflex is that it’s triggered even when just your face comes in contact with water! A big splash to the face activates receptors in your nose and sinus cavity, and that triggers a physiological override via your trigeminal nerve, which is the largest cranial nerve.

This sends an immediate “hey-we’re-underwater-water-now” message to the brain, and these physiological changes start to kick in. So next time you feel stressed out, splash some water over your face, or go for a dip! You can thank the mammalian dive reflex for the chill vibes that come along with a nice soak.

If you want to learn more about underwater science, you can check out our episode about whether humans could ever breathe underwater. And as always, thanks for watching this episode of SciShow! [ outro ].