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Hank: Are people constantly asking you if you got enough sleep last night, even if you snoozed for a solid eight hours?

Well, look alive! Today, we're gonna talk about the science behind those dark circles under your eyes and all the things that might cause them, tiredness included.

There's a simple reason those eye-bags look all purplish: just check out your wrists. If you can see the veins under your skin, they might look blue, even though the blood inside of them is red. That's because of the way that different wavelengths of light interact with your skin and blood vessels, so more blue light is reflected back to our eyes. The area under your eyes appears darker because the skin is thinner there, and doesn't have as much fat. That means that more tiny blood vessels can show through.

People with darker skin have more pigment, called melanin, in their skin, and sometimes they produce extra melanin below their eyes, a condition called periorbital hyperpigmentation. So, based on the thickness of your skin and genetics, you might be stuck with any dark circles you're born with, but there are some things you can do to make them worse.

People usually blame their dark circles on tiredness, and there probably is a correlation. See, when you're tired or stressed, your body produces more of a hormone called cortisol to get more glucose into your blood stream to give you the energy to stay awake. Over time, cortisol constricts some blood vessels and dilates others, so it's possible that the vessels under your eyes are a little bigger, which means darker eye circles.

Another culprit is allergies, or any sort of congestion. Allergic reactions can trigger your immune system to release these compounds called histamines, which cause blood vessels to dilate. Plus, itching and rubbing your eyes can make the puffiness worse.

But besides the dark circles from built up blood and inflammation, puffy eye-bags can also mean dark shadows. Not to mention things like ultraviolet radiation in sunlight, which can damage the DNA in skin cells, which can kill them off.

And unfortunately, as we get older, we lose even more fat underneath our skin, and it gets thinner. Which means more, darker circles. So the best things you can do: wear some sunscreen, take your allergy medicine, get a good night's rest, and try not to sweat the small stuff.

Thanks to Patreon patrons Phillip Hollier-Day and Zul for asking, and thanks to all of our patrons who keep these answers coming. If you would like to submit a question to be answered, you can go to Patreon.com/SciShow, and don't forget to go to YouTube.com/SciShow and subscribe.

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